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Homework question about an engine pulling a train

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Homework question about an engine pulling a train



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Planned maintenance scheduled April 17/18, 2019 at 00:00UTC (8:00pm US/Eastern)Mechanics Question: Work and PowerMechanics Question: Energy, Work and PowerCan we define tension in a string as the reactive force produced in a string being pulled at both ends?What objects' masses do we need to take into account when calculating force?Does the frame of reference change the behavior of a particleA train is moving up an incline of inclination $pi/6$ with constant velocity. A bob is hung on the ceiling. The angle made by the bob with normal?Homework - Calculating normal force with additional accelerations (besides gravity)What does the Force in the equation P=FV represent?Infinite loop of forces?Mechanics Question - Thrust in a Rod between train and carriage












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An 8000 kg engine pulls a train of 5 wagons, each of 2000 kg, along a
horizontal track. If the engine exerts a force of 40000N , calculate the acceleration of the train.




To find the acceleration of the train , I divided 40000N by mass of 5 wagons.
But in the book the mass of the engine was also included for finding the acceleration.



I cannot understand how is the force of 40000 N acting on (engine + wagons). According to me this should only act on the wagons as the engine is applying this force.






homework-and-exercises newtonian-mechanics forces







share|cite|improve this question











$endgroup$








  • 6




    $begingroup$
    This is a poorly worded question. I read (like you) that the engine exerts 40000 N on the train of wagons. What they should have said is that the net force on the engine and train is 40000 N, if the mass of the engine was involved. The only correct answer is to give both situations. The book is wrong.
    $endgroup$
    – Bill N
    14 hours ago






  • 5




    $begingroup$
    It's tricky but it says "the engine exerts a force" it does not say " the engine exerts a force on the cars". And that's the reason you include the engine!
    $endgroup$
    – PhysicsDave
    13 hours ago






  • 2




    $begingroup$
    The problem is ambiguously worded, but I'm shocked that you have all forgotten to include the vertical force exerted by the weight of the engine on the ground! Surely, we're meant to subtract that out from the 40kN total force. Since the question does not specify what the local gravity is on the train's planet, we clearly cannot solve it.
    $endgroup$
    – Xerxes
    12 hours ago






  • 4




    $begingroup$
    @Xerxes This is completely wrong. The vertical forces cancel each other and have nothing to do with the horizontal force pulling the train. You forget the normal force exerted by the ground on the train.
    $endgroup$
    – nasu
    10 hours ago






  • 1




    $begingroup$
    @nasu The cancelling normal force is exerted on, not by, the engine.
    $endgroup$
    – Xerxes
    10 hours ago
















6












$begingroup$



An 8000 kg engine pulls a train of 5 wagons, each of 2000 kg, along a
horizontal track. If the engine exerts a force of 40000N , calculate the acceleration of the train.




To find the acceleration of the train , I divided 40000N by mass of 5 wagons.
But in the book the mass of the engine was also included for finding the acceleration.



I cannot understand how is the force of 40000 N acting on (engine + wagons). According to me this should only act on the wagons as the engine is applying this force.






homework-and-exercises newtonian-mechanics forces







share|cite|improve this question











$endgroup$








  • 6




    $begingroup$
    This is a poorly worded question. I read (like you) that the engine exerts 40000 N on the train of wagons. What they should have said is that the net force on the engine and train is 40000 N, if the mass of the engine was involved. The only correct answer is to give both situations. The book is wrong.
    $endgroup$
    – Bill N
    14 hours ago






  • 5




    $begingroup$
    It's tricky but it says "the engine exerts a force" it does not say " the engine exerts a force on the cars". And that's the reason you include the engine!
    $endgroup$
    – PhysicsDave
    13 hours ago






  • 2




    $begingroup$
    The problem is ambiguously worded, but I'm shocked that you have all forgotten to include the vertical force exerted by the weight of the engine on the ground! Surely, we're meant to subtract that out from the 40kN total force. Since the question does not specify what the local gravity is on the train's planet, we clearly cannot solve it.
    $endgroup$
    – Xerxes
    12 hours ago






  • 4




    $begingroup$
    @Xerxes This is completely wrong. The vertical forces cancel each other and have nothing to do with the horizontal force pulling the train. You forget the normal force exerted by the ground on the train.
    $endgroup$
    – nasu
    10 hours ago






  • 1




    $begingroup$
    @nasu The cancelling normal force is exerted on, not by, the engine.
    $endgroup$
    – Xerxes
    10 hours ago














6












6








6


1



$begingroup$



An 8000 kg engine pulls a train of 5 wagons, each of 2000 kg, along a
horizontal track. If the engine exerts a force of 40000N , calculate the acceleration of the train.




To find the acceleration of the train , I divided 40000N by mass of 5 wagons.
But in the book the mass of the engine was also included for finding the acceleration.



I cannot understand how is the force of 40000 N acting on (engine + wagons). According to me this should only act on the wagons as the engine is applying this force.






homework-and-exercises newtonian-mechanics forces







share|cite|improve this question











$endgroup$





An 8000 kg engine pulls a train of 5 wagons, each of 2000 kg, along a
horizontal track. If the engine exerts a force of 40000N , calculate the acceleration of the train.




To find the acceleration of the train , I divided 40000N by mass of 5 wagons.
But in the book the mass of the engine was also included for finding the acceleration.



I cannot understand how is the force of 40000 N acting on (engine + wagons). According to me this should only act on the wagons as the engine is applying this force.






homework-and-exercises newtonian-mechanics forces




homework-and-exercises newtonian-mechanics forces






share|cite|improve this question















share|cite|improve this question













share|cite|improve this question




share|cite|improve this question








edited 8 hours ago









knzhou

47k11127226




47k11127226










asked 15 hours ago









Ashok SharmaAshok Sharma

708




708








  • 6




    $begingroup$
    This is a poorly worded question. I read (like you) that the engine exerts 40000 N on the train of wagons. What they should have said is that the net force on the engine and train is 40000 N, if the mass of the engine was involved. The only correct answer is to give both situations. The book is wrong.
    $endgroup$
    – Bill N
    14 hours ago






  • 5




    $begingroup$
    It's tricky but it says "the engine exerts a force" it does not say " the engine exerts a force on the cars". And that's the reason you include the engine!
    $endgroup$
    – PhysicsDave
    13 hours ago






  • 2




    $begingroup$
    The problem is ambiguously worded, but I'm shocked that you have all forgotten to include the vertical force exerted by the weight of the engine on the ground! Surely, we're meant to subtract that out from the 40kN total force. Since the question does not specify what the local gravity is on the train's planet, we clearly cannot solve it.
    $endgroup$
    – Xerxes
    12 hours ago






  • 4




    $begingroup$
    @Xerxes This is completely wrong. The vertical forces cancel each other and have nothing to do with the horizontal force pulling the train. You forget the normal force exerted by the ground on the train.
    $endgroup$
    – nasu
    10 hours ago






  • 1




    $begingroup$
    @nasu The cancelling normal force is exerted on, not by, the engine.
    $endgroup$
    – Xerxes
    10 hours ago














  • 6




    $begingroup$
    This is a poorly worded question. I read (like you) that the engine exerts 40000 N on the train of wagons. What they should have said is that the net force on the engine and train is 40000 N, if the mass of the engine was involved. The only correct answer is to give both situations. The book is wrong.
    $endgroup$
    – Bill N
    14 hours ago






  • 5




    $begingroup$
    It's tricky but it says "the engine exerts a force" it does not say " the engine exerts a force on the cars". And that's the reason you include the engine!
    $endgroup$
    – PhysicsDave
    13 hours ago






  • 2




    $begingroup$
    The problem is ambiguously worded, but I'm shocked that you have all forgotten to include the vertical force exerted by the weight of the engine on the ground! Surely, we're meant to subtract that out from the 40kN total force. Since the question does not specify what the local gravity is on the train's planet, we clearly cannot solve it.
    $endgroup$
    – Xerxes
    12 hours ago






  • 4




    $begingroup$
    @Xerxes This is completely wrong. The vertical forces cancel each other and have nothing to do with the horizontal force pulling the train. You forget the normal force exerted by the ground on the train.
    $endgroup$
    – nasu
    10 hours ago






  • 1




    $begingroup$
    @nasu The cancelling normal force is exerted on, not by, the engine.
    $endgroup$
    – Xerxes
    10 hours ago








6




6




$begingroup$
This is a poorly worded question. I read (like you) that the engine exerts 40000 N on the train of wagons. What they should have said is that the net force on the engine and train is 40000 N, if the mass of the engine was involved. The only correct answer is to give both situations. The book is wrong.
$endgroup$
– Bill N
14 hours ago




$begingroup$
This is a poorly worded question. I read (like you) that the engine exerts 40000 N on the train of wagons. What they should have said is that the net force on the engine and train is 40000 N, if the mass of the engine was involved. The only correct answer is to give both situations. The book is wrong.
$endgroup$
– Bill N
14 hours ago




5




5




$begingroup$
It's tricky but it says "the engine exerts a force" it does not say " the engine exerts a force on the cars". And that's the reason you include the engine!
$endgroup$
– PhysicsDave
13 hours ago




$begingroup$
It's tricky but it says "the engine exerts a force" it does not say " the engine exerts a force on the cars". And that's the reason you include the engine!
$endgroup$
– PhysicsDave
13 hours ago




2




2




$begingroup$
The problem is ambiguously worded, but I'm shocked that you have all forgotten to include the vertical force exerted by the weight of the engine on the ground! Surely, we're meant to subtract that out from the 40kN total force. Since the question does not specify what the local gravity is on the train's planet, we clearly cannot solve it.
$endgroup$
– Xerxes
12 hours ago




$begingroup$
The problem is ambiguously worded, but I'm shocked that you have all forgotten to include the vertical force exerted by the weight of the engine on the ground! Surely, we're meant to subtract that out from the 40kN total force. Since the question does not specify what the local gravity is on the train's planet, we clearly cannot solve it.
$endgroup$
– Xerxes
12 hours ago




4




4




$begingroup$
@Xerxes This is completely wrong. The vertical forces cancel each other and have nothing to do with the horizontal force pulling the train. You forget the normal force exerted by the ground on the train.
$endgroup$
– nasu
10 hours ago




$begingroup$
@Xerxes This is completely wrong. The vertical forces cancel each other and have nothing to do with the horizontal force pulling the train. You forget the normal force exerted by the ground on the train.
$endgroup$
– nasu
10 hours ago




1




1




$begingroup$
@nasu The cancelling normal force is exerted on, not by, the engine.
$endgroup$
– Xerxes
10 hours ago




$begingroup$
@nasu The cancelling normal force is exerted on, not by, the engine.
$endgroup$
– Xerxes
10 hours ago










6 Answers
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The question is ambiguous, but I think we are meant to assume that the engine exerts a force of $40000$ N on the track. From the reaction force of $40000$ N on the engine some part, say $F_1$, is acting to accelerate the engine itself and some other part, say $F_2$, is exerted on the wagons. The acceleration of the engine and of the wagons is the same, so if this acceleration is $a$ then



$F_1 = 8000a \ F_2 = 10000a \ 40000 = F_1 + F_2 = 18000a \ Rightarrow a = frac{40000}{18000}$






share|cite|improve this answer











$endgroup$









  • 1




    $begingroup$
    I agree, the wording includes the engine.
    $endgroup$
    – PhysicsDave
    13 hours ago










  • $begingroup$
    I would argue that the engine exerts the force on the track, and the reaction force is what accelerates the (whole) train (engine+wagons). Otherwise the problem would have specified the force acting at the first coupling, or used some indication like "net".
    $endgroup$
    – CCTO
    10 hours ago










  • $begingroup$
    @CCTO Agreed - I will add this to my answer.
    $endgroup$
    – gandalf61
    10 hours ago










  • $begingroup$
    The OP's question is a good one, but I don't think that the test question is ambiguous in the slightest. It seems to me that the OP's misunderstanding of it is exactly the sort of misunderstanding that this question is designed to test.
    $endgroup$
    – bornfromanegg
    9 hours ago








  • 1




    $begingroup$
    @bornfromanegg I think the ambiguity comes from the word "engine". if we assume that the engine is a moving engine i.e. a locomotive then the $40000$ N is accelerating both the engine and the wagons. But "engine" could possibly mean stationary engine, in which case the $40000$ N is accelerating only the wagons and the mass of the engine is irrelevant.
    $endgroup$
    – gandalf61
    9 hours ago



















5












$begingroup$

IMO you're right: the problem's wording is bad. It's meaningless to say "the engine exerts a force of 40000 N". The engine interacts both with the first wagon and with the rail, exerting forces on both and these forces are different. How can we know what the book's author had in mind? If the former, your answer is right, if the latter, the book's one is right. Of course in the latter case (force exerted on rail) it's not that force which accelerates the train, but the rail's reaction.



As a general rule we shouldn't say "A exerts a force" - much better "A exerts a force on B". Better yet "A exerts a force on point P of B". The point where a force acts can make a great difference.



Edit



Many things have been said but IMO it's useful to be still clearer. After further reflections I think it's not wording that's bad, but the thinking behind.



As already stated e.g. by @Farcher in mechanics problems the first thing to be done is to make clear which the (mechanical) system is. In our case, it's wagons or wagons + engine? Note that it wouldn't make
difference as to acceleration, which is the same for all parts of the train, but isn't by no means the same if forces are concerned. Consider the two choices:



a) The system consists of wagons alone. Then it makes sense to say that engine (which doesn't belong to the system) applies a given force $F$ to the system and compute acceleration as $a=F/m$, where
$m=10000,rm kg$. Given the book's answer, this isn't the author's choice.



b) The system consists of the whole train, engine included. Then there are two alternatives as to the given force $F$ (here I'm reiterating)



b1) $F$ is a force engine applies to wagons. Then it's an internal force and as such it cannot accelerate the system. Its acceleration is only due to the net external force (in computing the resultant internal forces cancel thanks to third Newton's law). Then the
solution is nonsense.



b2) $F$ is a force engine applies to external world. In that case it will never accelerate the system or a part of it. It could accelerate the rail, were it not firmly fixed to ground. Not to mention that a force engine applies to rail exists, but is in the wrong direction.



So not even choice b) is acceptable. Neither as it's used in gandalf61's answer.



I'd like to add another comment. Unfortunately "engine" in English may mean two different things: either the motor proper (electric, diesel,
or else) which ultimately is the cause of motion - or the car carrying it. Of course the datum of mass suggests the latter interpretation. But my suspicion is that the author - maybe without realizing - thought
of the former.



I'm led to say so because it's a very common way of thinking about cars and other motorized vehicles. If you ask someone "which force causes a car move?" I bet you'll get as an answer "the motor's force". Nobody thinks that in strict mechanical meaning the motor can only produce internal forces and as such will never move a car by a cm. That the only external force which can put a car in motion is the road's friction on the wheels.






share|cite|improve this answer











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    4












    $begingroup$

    You have to add mass of engine also.The force is applied on whole system.What will be the acceleration of only engine if you remove wagons?You will consider it's mass also.






    share|cite|improve this answer








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    Broly is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
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    $endgroup$









    • 5




      $begingroup$
      "The force is applied on whole system." That's NOT what the words of the problem say. The engine exerts a force---that must be on something else. In this type of analysis, things don't have self-forces. It's a terrible question.
      $endgroup$
      – Bill N
      13 hours ago










    • $begingroup$
      @Bill N: The engine exerts a force on the TRACK, which causes it to accelerate. Since the engine is connected to the cars, it must accelerate them as well, so the mass being accelerated is engine + cars.
      $endgroup$
      – jamesqf
      10 hours ago










    • $begingroup$
      @jamesqf The engine also exerts a force on the tracks due to gravity. Since we were just given total force exerted by the engine on the tracks, don't forget to subtract the gravity portion when calculating the force accelerating the train.
      $endgroup$
      – Matt
      9 hours ago










    • $begingroup$
      @Matt As long as we're being pedantic, they don't say which planet this train is on, or if it's outer space, so the gravitational force is undefined and the question is unsolvable.
      $endgroup$
      – Nuclear Wang
      7 hours ago










    • $begingroup$
      @NuclearWang Good point! But I disagree that accounting for gravity is pedantic. (Anyway, assuming earth is a pretty standard assumption for entry level physics) The question doesnt specify what the 40kN is exerted on. It could reasonably be on the first wagon in the line, or it could be the tracks. But in fact neither of these are what the "correct" answer assumes. I dont think its fair to expect arbitrary assumptions while also expecting the OP to ignore a very real, very important, force. Especially since the engine apparently exerts a net 40 kN on presisely zero objects.
      $endgroup$
      – Matt
      7 hours ago



















    3












    $begingroup$

    Imagine if you only had the engine by itself exerting the same 40,000N of thrust, to find the acceleration of the engine you would not do 40000/0kg would you? The 40,000N the engine exert is also used to thrust itself forward.



    But let’s say it didn’t, and that only the wagons were pulled. Well now you have a stationary engine, pulling 5 wagons, each with an acceleration of 4m/s^2. These wagons will catch up to the engine in no time and obliterate the engine from behind. we can at least agree this is not how train works






    share|cite|improve this answer








    New contributor




    Ubaid Hassan is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
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    • $begingroup$
      If the engine is by itself,, what is exerting thrust on? It can't exert thrust on itself. There must be something else exerting thrust on the engine. Regarding your second paragraph, there is obviously some force acting on the engine, but we aren't told what it is. All we know is that the engine exerts a force of 40000 N, presumably on the train of wagons.
      $endgroup$
      – Bill N
      13 hours ago






    • 1




      $begingroup$
      It's tricky but it says "the engine exerts a force" it does not say " the engine exerts a force on the cars". And that's the reason you include the engine! The problem is solvable for either situation.
      $endgroup$
      – PhysicsDave
      13 hours ago










    • $begingroup$
      I do agree with Physics Dave. Though the very existence of this Q and some comments tell that the exercise could have been written more clearly.
      $endgroup$
      – Alchimista
      11 hours ago










    • $begingroup$
      @BillN on the rails, naturally.
      $endgroup$
      – hobbs
      10 hours ago





















    1












    $begingroup$

    The question is flawed in that can be deemed to be ambiguous and therefore requires interpretation.



    Using Newton’s second law requires the system under consideration to be defined and that has not been done in this question.



    Without the system being defined the statement




    the engine exerts a force of 40000N




    is the cause of the ambiguity.



    So which is it?



    1 The engine is exerting an external force of 40000N on the system of five wagons.



    2 The engine exerts an internal force of 40000N on the system of five wagons and the engine.



    3 There is an external force of 40000N on the engine and five wagons.



    The author of the book really meant option 3 but given the way the question was written I have to agree with the OP that an external force of 40000N is acting on the five wagons whcich is option 1.






    share|cite|improve this answer











    $endgroup$





















      0












      $begingroup$

      The force of 40 kN is exerted on the rails, not the wagons



      We have to assume that the engine and wagons are all coupled together, and therefore that they move together. If the engine exerted a force on the wagons but did not move with them, the wagons would provide a different force in reaction. If the train was off the track, it would not be exerting that force on the wagons.



      The engine turns the wheels which exert a force backwards on the rails, which also imparts the reaction forwards on the train.



      That force is transmitted from the rails through the engine and the coupling to the wagons.



      To take a different circumstance, suppose the train was going uphill? The engine will still exert a force of the same magnitude, but this time the balance of forces and angles are different, so the acceleration is different too.



      Thus that force must accelerate the total mass of the engine and the wagons together.






      share|cite|improve this answer









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        6 Answers
        6






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        6 Answers
        6






        active

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        $begingroup$

        The question is ambiguous, but I think we are meant to assume that the engine exerts a force of $40000$ N on the track. From the reaction force of $40000$ N on the engine some part, say $F_1$, is acting to accelerate the engine itself and some other part, say $F_2$, is exerted on the wagons. The acceleration of the engine and of the wagons is the same, so if this acceleration is $a$ then



        $F_1 = 8000a \ F_2 = 10000a \ 40000 = F_1 + F_2 = 18000a \ Rightarrow a = frac{40000}{18000}$






        share|cite|improve this answer











        $endgroup$









        • 1




          $begingroup$
          I agree, the wording includes the engine.
          $endgroup$
          – PhysicsDave
          13 hours ago










        • $begingroup$
          I would argue that the engine exerts the force on the track, and the reaction force is what accelerates the (whole) train (engine+wagons). Otherwise the problem would have specified the force acting at the first coupling, or used some indication like "net".
          $endgroup$
          – CCTO
          10 hours ago










        • $begingroup$
          @CCTO Agreed - I will add this to my answer.
          $endgroup$
          – gandalf61
          10 hours ago










        • $begingroup$
          The OP's question is a good one, but I don't think that the test question is ambiguous in the slightest. It seems to me that the OP's misunderstanding of it is exactly the sort of misunderstanding that this question is designed to test.
          $endgroup$
          – bornfromanegg
          9 hours ago








        • 1




          $begingroup$
          @bornfromanegg I think the ambiguity comes from the word "engine". if we assume that the engine is a moving engine i.e. a locomotive then the $40000$ N is accelerating both the engine and the wagons. But "engine" could possibly mean stationary engine, in which case the $40000$ N is accelerating only the wagons and the mass of the engine is irrelevant.
          $endgroup$
          – gandalf61
          9 hours ago
















        9












        $begingroup$

        The question is ambiguous, but I think we are meant to assume that the engine exerts a force of $40000$ N on the track. From the reaction force of $40000$ N on the engine some part, say $F_1$, is acting to accelerate the engine itself and some other part, say $F_2$, is exerted on the wagons. The acceleration of the engine and of the wagons is the same, so if this acceleration is $a$ then



        $F_1 = 8000a \ F_2 = 10000a \ 40000 = F_1 + F_2 = 18000a \ Rightarrow a = frac{40000}{18000}$






        share|cite|improve this answer











        $endgroup$









        • 1




          $begingroup$
          I agree, the wording includes the engine.
          $endgroup$
          – PhysicsDave
          13 hours ago










        • $begingroup$
          I would argue that the engine exerts the force on the track, and the reaction force is what accelerates the (whole) train (engine+wagons). Otherwise the problem would have specified the force acting at the first coupling, or used some indication like "net".
          $endgroup$
          – CCTO
          10 hours ago










        • $begingroup$
          @CCTO Agreed - I will add this to my answer.
          $endgroup$
          – gandalf61
          10 hours ago










        • $begingroup$
          The OP's question is a good one, but I don't think that the test question is ambiguous in the slightest. It seems to me that the OP's misunderstanding of it is exactly the sort of misunderstanding that this question is designed to test.
          $endgroup$
          – bornfromanegg
          9 hours ago








        • 1




          $begingroup$
          @bornfromanegg I think the ambiguity comes from the word "engine". if we assume that the engine is a moving engine i.e. a locomotive then the $40000$ N is accelerating both the engine and the wagons. But "engine" could possibly mean stationary engine, in which case the $40000$ N is accelerating only the wagons and the mass of the engine is irrelevant.
          $endgroup$
          – gandalf61
          9 hours ago














        9












        9








        9





        $begingroup$

        The question is ambiguous, but I think we are meant to assume that the engine exerts a force of $40000$ N on the track. From the reaction force of $40000$ N on the engine some part, say $F_1$, is acting to accelerate the engine itself and some other part, say $F_2$, is exerted on the wagons. The acceleration of the engine and of the wagons is the same, so if this acceleration is $a$ then



        $F_1 = 8000a \ F_2 = 10000a \ 40000 = F_1 + F_2 = 18000a \ Rightarrow a = frac{40000}{18000}$






        share|cite|improve this answer











        $endgroup$



        The question is ambiguous, but I think we are meant to assume that the engine exerts a force of $40000$ N on the track. From the reaction force of $40000$ N on the engine some part, say $F_1$, is acting to accelerate the engine itself and some other part, say $F_2$, is exerted on the wagons. The acceleration of the engine and of the wagons is the same, so if this acceleration is $a$ then



        $F_1 = 8000a \ F_2 = 10000a \ 40000 = F_1 + F_2 = 18000a \ Rightarrow a = frac{40000}{18000}$







        share|cite|improve this answer














        share|cite|improve this answer



        share|cite|improve this answer








        edited 9 hours ago

























        answered 14 hours ago









        gandalf61gandalf61

        63229




        63229








        • 1




          $begingroup$
          I agree, the wording includes the engine.
          $endgroup$
          – PhysicsDave
          13 hours ago










        • $begingroup$
          I would argue that the engine exerts the force on the track, and the reaction force is what accelerates the (whole) train (engine+wagons). Otherwise the problem would have specified the force acting at the first coupling, or used some indication like "net".
          $endgroup$
          – CCTO
          10 hours ago










        • $begingroup$
          @CCTO Agreed - I will add this to my answer.
          $endgroup$
          – gandalf61
          10 hours ago










        • $begingroup$
          The OP's question is a good one, but I don't think that the test question is ambiguous in the slightest. It seems to me that the OP's misunderstanding of it is exactly the sort of misunderstanding that this question is designed to test.
          $endgroup$
          – bornfromanegg
          9 hours ago








        • 1




          $begingroup$
          @bornfromanegg I think the ambiguity comes from the word "engine". if we assume that the engine is a moving engine i.e. a locomotive then the $40000$ N is accelerating both the engine and the wagons. But "engine" could possibly mean stationary engine, in which case the $40000$ N is accelerating only the wagons and the mass of the engine is irrelevant.
          $endgroup$
          – gandalf61
          9 hours ago














        • 1




          $begingroup$
          I agree, the wording includes the engine.
          $endgroup$
          – PhysicsDave
          13 hours ago










        • $begingroup$
          I would argue that the engine exerts the force on the track, and the reaction force is what accelerates the (whole) train (engine+wagons). Otherwise the problem would have specified the force acting at the first coupling, or used some indication like "net".
          $endgroup$
          – CCTO
          10 hours ago










        • $begingroup$
          @CCTO Agreed - I will add this to my answer.
          $endgroup$
          – gandalf61
          10 hours ago










        • $begingroup$
          The OP's question is a good one, but I don't think that the test question is ambiguous in the slightest. It seems to me that the OP's misunderstanding of it is exactly the sort of misunderstanding that this question is designed to test.
          $endgroup$
          – bornfromanegg
          9 hours ago








        • 1




          $begingroup$
          @bornfromanegg I think the ambiguity comes from the word "engine". if we assume that the engine is a moving engine i.e. a locomotive then the $40000$ N is accelerating both the engine and the wagons. But "engine" could possibly mean stationary engine, in which case the $40000$ N is accelerating only the wagons and the mass of the engine is irrelevant.
          $endgroup$
          – gandalf61
          9 hours ago








        1




        1




        $begingroup$
        I agree, the wording includes the engine.
        $endgroup$
        – PhysicsDave
        13 hours ago




        $begingroup$
        I agree, the wording includes the engine.
        $endgroup$
        – PhysicsDave
        13 hours ago












        $begingroup$
        I would argue that the engine exerts the force on the track, and the reaction force is what accelerates the (whole) train (engine+wagons). Otherwise the problem would have specified the force acting at the first coupling, or used some indication like "net".
        $endgroup$
        – CCTO
        10 hours ago




        $begingroup$
        I would argue that the engine exerts the force on the track, and the reaction force is what accelerates the (whole) train (engine+wagons). Otherwise the problem would have specified the force acting at the first coupling, or used some indication like "net".
        $endgroup$
        – CCTO
        10 hours ago












        $begingroup$
        @CCTO Agreed - I will add this to my answer.
        $endgroup$
        – gandalf61
        10 hours ago




        $begingroup$
        @CCTO Agreed - I will add this to my answer.
        $endgroup$
        – gandalf61
        10 hours ago












        $begingroup$
        The OP's question is a good one, but I don't think that the test question is ambiguous in the slightest. It seems to me that the OP's misunderstanding of it is exactly the sort of misunderstanding that this question is designed to test.
        $endgroup$
        – bornfromanegg
        9 hours ago






        $begingroup$
        The OP's question is a good one, but I don't think that the test question is ambiguous in the slightest. It seems to me that the OP's misunderstanding of it is exactly the sort of misunderstanding that this question is designed to test.
        $endgroup$
        – bornfromanegg
        9 hours ago






        1




        1




        $begingroup$
        @bornfromanegg I think the ambiguity comes from the word "engine". if we assume that the engine is a moving engine i.e. a locomotive then the $40000$ N is accelerating both the engine and the wagons. But "engine" could possibly mean stationary engine, in which case the $40000$ N is accelerating only the wagons and the mass of the engine is irrelevant.
        $endgroup$
        – gandalf61
        9 hours ago




        $begingroup$
        @bornfromanegg I think the ambiguity comes from the word "engine". if we assume that the engine is a moving engine i.e. a locomotive then the $40000$ N is accelerating both the engine and the wagons. But "engine" could possibly mean stationary engine, in which case the $40000$ N is accelerating only the wagons and the mass of the engine is irrelevant.
        $endgroup$
        – gandalf61
        9 hours ago











        5












        $begingroup$

        IMO you're right: the problem's wording is bad. It's meaningless to say "the engine exerts a force of 40000 N". The engine interacts both with the first wagon and with the rail, exerting forces on both and these forces are different. How can we know what the book's author had in mind? If the former, your answer is right, if the latter, the book's one is right. Of course in the latter case (force exerted on rail) it's not that force which accelerates the train, but the rail's reaction.



        As a general rule we shouldn't say "A exerts a force" - much better "A exerts a force on B". Better yet "A exerts a force on point P of B". The point where a force acts can make a great difference.



        Edit



        Many things have been said but IMO it's useful to be still clearer. After further reflections I think it's not wording that's bad, but the thinking behind.



        As already stated e.g. by @Farcher in mechanics problems the first thing to be done is to make clear which the (mechanical) system is. In our case, it's wagons or wagons + engine? Note that it wouldn't make
        difference as to acceleration, which is the same for all parts of the train, but isn't by no means the same if forces are concerned. Consider the two choices:



        a) The system consists of wagons alone. Then it makes sense to say that engine (which doesn't belong to the system) applies a given force $F$ to the system and compute acceleration as $a=F/m$, where
        $m=10000,rm kg$. Given the book's answer, this isn't the author's choice.



        b) The system consists of the whole train, engine included. Then there are two alternatives as to the given force $F$ (here I'm reiterating)



        b1) $F$ is a force engine applies to wagons. Then it's an internal force and as such it cannot accelerate the system. Its acceleration is only due to the net external force (in computing the resultant internal forces cancel thanks to third Newton's law). Then the
        solution is nonsense.



        b2) $F$ is a force engine applies to external world. In that case it will never accelerate the system or a part of it. It could accelerate the rail, were it not firmly fixed to ground. Not to mention that a force engine applies to rail exists, but is in the wrong direction.



        So not even choice b) is acceptable. Neither as it's used in gandalf61's answer.



        I'd like to add another comment. Unfortunately "engine" in English may mean two different things: either the motor proper (electric, diesel,
        or else) which ultimately is the cause of motion - or the car carrying it. Of course the datum of mass suggests the latter interpretation. But my suspicion is that the author - maybe without realizing - thought
        of the former.



        I'm led to say so because it's a very common way of thinking about cars and other motorized vehicles. If you ask someone "which force causes a car move?" I bet you'll get as an answer "the motor's force". Nobody thinks that in strict mechanical meaning the motor can only produce internal forces and as such will never move a car by a cm. That the only external force which can put a car in motion is the road's friction on the wheels.






        share|cite|improve this answer











        $endgroup$


















          5












          $begingroup$

          IMO you're right: the problem's wording is bad. It's meaningless to say "the engine exerts a force of 40000 N". The engine interacts both with the first wagon and with the rail, exerting forces on both and these forces are different. How can we know what the book's author had in mind? If the former, your answer is right, if the latter, the book's one is right. Of course in the latter case (force exerted on rail) it's not that force which accelerates the train, but the rail's reaction.



          As a general rule we shouldn't say "A exerts a force" - much better "A exerts a force on B". Better yet "A exerts a force on point P of B". The point where a force acts can make a great difference.



          Edit



          Many things have been said but IMO it's useful to be still clearer. After further reflections I think it's not wording that's bad, but the thinking behind.



          As already stated e.g. by @Farcher in mechanics problems the first thing to be done is to make clear which the (mechanical) system is. In our case, it's wagons or wagons + engine? Note that it wouldn't make
          difference as to acceleration, which is the same for all parts of the train, but isn't by no means the same if forces are concerned. Consider the two choices:



          a) The system consists of wagons alone. Then it makes sense to say that engine (which doesn't belong to the system) applies a given force $F$ to the system and compute acceleration as $a=F/m$, where
          $m=10000,rm kg$. Given the book's answer, this isn't the author's choice.



          b) The system consists of the whole train, engine included. Then there are two alternatives as to the given force $F$ (here I'm reiterating)



          b1) $F$ is a force engine applies to wagons. Then it's an internal force and as such it cannot accelerate the system. Its acceleration is only due to the net external force (in computing the resultant internal forces cancel thanks to third Newton's law). Then the
          solution is nonsense.



          b2) $F$ is a force engine applies to external world. In that case it will never accelerate the system or a part of it. It could accelerate the rail, were it not firmly fixed to ground. Not to mention that a force engine applies to rail exists, but is in the wrong direction.



          So not even choice b) is acceptable. Neither as it's used in gandalf61's answer.



          I'd like to add another comment. Unfortunately "engine" in English may mean two different things: either the motor proper (electric, diesel,
          or else) which ultimately is the cause of motion - or the car carrying it. Of course the datum of mass suggests the latter interpretation. But my suspicion is that the author - maybe without realizing - thought
          of the former.



          I'm led to say so because it's a very common way of thinking about cars and other motorized vehicles. If you ask someone "which force causes a car move?" I bet you'll get as an answer "the motor's force". Nobody thinks that in strict mechanical meaning the motor can only produce internal forces and as such will never move a car by a cm. That the only external force which can put a car in motion is the road's friction on the wheels.






          share|cite|improve this answer











          $endgroup$
















            5












            5








            5





            $begingroup$

            IMO you're right: the problem's wording is bad. It's meaningless to say "the engine exerts a force of 40000 N". The engine interacts both with the first wagon and with the rail, exerting forces on both and these forces are different. How can we know what the book's author had in mind? If the former, your answer is right, if the latter, the book's one is right. Of course in the latter case (force exerted on rail) it's not that force which accelerates the train, but the rail's reaction.



            As a general rule we shouldn't say "A exerts a force" - much better "A exerts a force on B". Better yet "A exerts a force on point P of B". The point where a force acts can make a great difference.



            Edit



            Many things have been said but IMO it's useful to be still clearer. After further reflections I think it's not wording that's bad, but the thinking behind.



            As already stated e.g. by @Farcher in mechanics problems the first thing to be done is to make clear which the (mechanical) system is. In our case, it's wagons or wagons + engine? Note that it wouldn't make
            difference as to acceleration, which is the same for all parts of the train, but isn't by no means the same if forces are concerned. Consider the two choices:



            a) The system consists of wagons alone. Then it makes sense to say that engine (which doesn't belong to the system) applies a given force $F$ to the system and compute acceleration as $a=F/m$, where
            $m=10000,rm kg$. Given the book's answer, this isn't the author's choice.



            b) The system consists of the whole train, engine included. Then there are two alternatives as to the given force $F$ (here I'm reiterating)



            b1) $F$ is a force engine applies to wagons. Then it's an internal force and as such it cannot accelerate the system. Its acceleration is only due to the net external force (in computing the resultant internal forces cancel thanks to third Newton's law). Then the
            solution is nonsense.



            b2) $F$ is a force engine applies to external world. In that case it will never accelerate the system or a part of it. It could accelerate the rail, were it not firmly fixed to ground. Not to mention that a force engine applies to rail exists, but is in the wrong direction.



            So not even choice b) is acceptable. Neither as it's used in gandalf61's answer.



            I'd like to add another comment. Unfortunately "engine" in English may mean two different things: either the motor proper (electric, diesel,
            or else) which ultimately is the cause of motion - or the car carrying it. Of course the datum of mass suggests the latter interpretation. But my suspicion is that the author - maybe without realizing - thought
            of the former.



            I'm led to say so because it's a very common way of thinking about cars and other motorized vehicles. If you ask someone "which force causes a car move?" I bet you'll get as an answer "the motor's force". Nobody thinks that in strict mechanical meaning the motor can only produce internal forces and as such will never move a car by a cm. That the only external force which can put a car in motion is the road's friction on the wheels.






            share|cite|improve this answer











            $endgroup$



            IMO you're right: the problem's wording is bad. It's meaningless to say "the engine exerts a force of 40000 N". The engine interacts both with the first wagon and with the rail, exerting forces on both and these forces are different. How can we know what the book's author had in mind? If the former, your answer is right, if the latter, the book's one is right. Of course in the latter case (force exerted on rail) it's not that force which accelerates the train, but the rail's reaction.



            As a general rule we shouldn't say "A exerts a force" - much better "A exerts a force on B". Better yet "A exerts a force on point P of B". The point where a force acts can make a great difference.



            Edit



            Many things have been said but IMO it's useful to be still clearer. After further reflections I think it's not wording that's bad, but the thinking behind.



            As already stated e.g. by @Farcher in mechanics problems the first thing to be done is to make clear which the (mechanical) system is. In our case, it's wagons or wagons + engine? Note that it wouldn't make
            difference as to acceleration, which is the same for all parts of the train, but isn't by no means the same if forces are concerned. Consider the two choices:



            a) The system consists of wagons alone. Then it makes sense to say that engine (which doesn't belong to the system) applies a given force $F$ to the system and compute acceleration as $a=F/m$, where
            $m=10000,rm kg$. Given the book's answer, this isn't the author's choice.



            b) The system consists of the whole train, engine included. Then there are two alternatives as to the given force $F$ (here I'm reiterating)



            b1) $F$ is a force engine applies to wagons. Then it's an internal force and as such it cannot accelerate the system. Its acceleration is only due to the net external force (in computing the resultant internal forces cancel thanks to third Newton's law). Then the
            solution is nonsense.



            b2) $F$ is a force engine applies to external world. In that case it will never accelerate the system or a part of it. It could accelerate the rail, were it not firmly fixed to ground. Not to mention that a force engine applies to rail exists, but is in the wrong direction.



            So not even choice b) is acceptable. Neither as it's used in gandalf61's answer.



            I'd like to add another comment. Unfortunately "engine" in English may mean two different things: either the motor proper (electric, diesel,
            or else) which ultimately is the cause of motion - or the car carrying it. Of course the datum of mass suggests the latter interpretation. But my suspicion is that the author - maybe without realizing - thought
            of the former.



            I'm led to say so because it's a very common way of thinking about cars and other motorized vehicles. If you ask someone "which force causes a car move?" I bet you'll get as an answer "the motor's force". Nobody thinks that in strict mechanical meaning the motor can only produce internal forces and as such will never move a car by a cm. That the only external force which can put a car in motion is the road's friction on the wheels.







            share|cite|improve this answer














            share|cite|improve this answer



            share|cite|improve this answer








            edited 12 hours ago

























            answered 13 hours ago









            Elio FabriElio Fabri

            3,6581214




            3,6581214























                4












                $begingroup$

                You have to add mass of engine also.The force is applied on whole system.What will be the acceleration of only engine if you remove wagons?You will consider it's mass also.






                share|cite|improve this answer








                New contributor




                Broly is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
                Check out our Code of Conduct.






                $endgroup$









                • 5




                  $begingroup$
                  "The force is applied on whole system." That's NOT what the words of the problem say. The engine exerts a force---that must be on something else. In this type of analysis, things don't have self-forces. It's a terrible question.
                  $endgroup$
                  – Bill N
                  13 hours ago










                • $begingroup$
                  @Bill N: The engine exerts a force on the TRACK, which causes it to accelerate. Since the engine is connected to the cars, it must accelerate them as well, so the mass being accelerated is engine + cars.
                  $endgroup$
                  – jamesqf
                  10 hours ago










                • $begingroup$
                  @jamesqf The engine also exerts a force on the tracks due to gravity. Since we were just given total force exerted by the engine on the tracks, don't forget to subtract the gravity portion when calculating the force accelerating the train.
                  $endgroup$
                  – Matt
                  9 hours ago










                • $begingroup$
                  @Matt As long as we're being pedantic, they don't say which planet this train is on, or if it's outer space, so the gravitational force is undefined and the question is unsolvable.
                  $endgroup$
                  – Nuclear Wang
                  7 hours ago










                • $begingroup$
                  @NuclearWang Good point! But I disagree that accounting for gravity is pedantic. (Anyway, assuming earth is a pretty standard assumption for entry level physics) The question doesnt specify what the 40kN is exerted on. It could reasonably be on the first wagon in the line, or it could be the tracks. But in fact neither of these are what the "correct" answer assumes. I dont think its fair to expect arbitrary assumptions while also expecting the OP to ignore a very real, very important, force. Especially since the engine apparently exerts a net 40 kN on presisely zero objects.
                  $endgroup$
                  – Matt
                  7 hours ago
















                4












                $begingroup$

                You have to add mass of engine also.The force is applied on whole system.What will be the acceleration of only engine if you remove wagons?You will consider it's mass also.






                share|cite|improve this answer








                New contributor




                Broly is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
                Check out our Code of Conduct.






                $endgroup$









                • 5




                  $begingroup$
                  "The force is applied on whole system." That's NOT what the words of the problem say. The engine exerts a force---that must be on something else. In this type of analysis, things don't have self-forces. It's a terrible question.
                  $endgroup$
                  – Bill N
                  13 hours ago










                • $begingroup$
                  @Bill N: The engine exerts a force on the TRACK, which causes it to accelerate. Since the engine is connected to the cars, it must accelerate them as well, so the mass being accelerated is engine + cars.
                  $endgroup$
                  – jamesqf
                  10 hours ago










                • $begingroup$
                  @jamesqf The engine also exerts a force on the tracks due to gravity. Since we were just given total force exerted by the engine on the tracks, don't forget to subtract the gravity portion when calculating the force accelerating the train.
                  $endgroup$
                  – Matt
                  9 hours ago










                • $begingroup$
                  @Matt As long as we're being pedantic, they don't say which planet this train is on, or if it's outer space, so the gravitational force is undefined and the question is unsolvable.
                  $endgroup$
                  – Nuclear Wang
                  7 hours ago










                • $begingroup$
                  @NuclearWang Good point! But I disagree that accounting for gravity is pedantic. (Anyway, assuming earth is a pretty standard assumption for entry level physics) The question doesnt specify what the 40kN is exerted on. It could reasonably be on the first wagon in the line, or it could be the tracks. But in fact neither of these are what the "correct" answer assumes. I dont think its fair to expect arbitrary assumptions while also expecting the OP to ignore a very real, very important, force. Especially since the engine apparently exerts a net 40 kN on presisely zero objects.
                  $endgroup$
                  – Matt
                  7 hours ago














                4












                4








                4





                $begingroup$

                You have to add mass of engine also.The force is applied on whole system.What will be the acceleration of only engine if you remove wagons?You will consider it's mass also.






                share|cite|improve this answer








                New contributor




                Broly is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
                Check out our Code of Conduct.






                $endgroup$



                You have to add mass of engine also.The force is applied on whole system.What will be the acceleration of only engine if you remove wagons?You will consider it's mass also.







                share|cite|improve this answer








                New contributor




                Broly is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
                Check out our Code of Conduct.









                share|cite|improve this answer



                share|cite|improve this answer






                New contributor




                Broly is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
                Check out our Code of Conduct.









                answered 15 hours ago









                BrolyBroly

                411113




                411113




                New contributor




                Broly is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
                Check out our Code of Conduct.





                New contributor





                Broly is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
                Check out our Code of Conduct.






                Broly is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
                Check out our Code of Conduct.








                • 5




                  $begingroup$
                  "The force is applied on whole system." That's NOT what the words of the problem say. The engine exerts a force---that must be on something else. In this type of analysis, things don't have self-forces. It's a terrible question.
                  $endgroup$
                  – Bill N
                  13 hours ago










                • $begingroup$
                  @Bill N: The engine exerts a force on the TRACK, which causes it to accelerate. Since the engine is connected to the cars, it must accelerate them as well, so the mass being accelerated is engine + cars.
                  $endgroup$
                  – jamesqf
                  10 hours ago










                • $begingroup$
                  @jamesqf The engine also exerts a force on the tracks due to gravity. Since we were just given total force exerted by the engine on the tracks, don't forget to subtract the gravity portion when calculating the force accelerating the train.
                  $endgroup$
                  – Matt
                  9 hours ago










                • $begingroup$
                  @Matt As long as we're being pedantic, they don't say which planet this train is on, or if it's outer space, so the gravitational force is undefined and the question is unsolvable.
                  $endgroup$
                  – Nuclear Wang
                  7 hours ago










                • $begingroup$
                  @NuclearWang Good point! But I disagree that accounting for gravity is pedantic. (Anyway, assuming earth is a pretty standard assumption for entry level physics) The question doesnt specify what the 40kN is exerted on. It could reasonably be on the first wagon in the line, or it could be the tracks. But in fact neither of these are what the "correct" answer assumes. I dont think its fair to expect arbitrary assumptions while also expecting the OP to ignore a very real, very important, force. Especially since the engine apparently exerts a net 40 kN on presisely zero objects.
                  $endgroup$
                  – Matt
                  7 hours ago














                • 5




                  $begingroup$
                  "The force is applied on whole system." That's NOT what the words of the problem say. The engine exerts a force---that must be on something else. In this type of analysis, things don't have self-forces. It's a terrible question.
                  $endgroup$
                  – Bill N
                  13 hours ago










                • $begingroup$
                  @Bill N: The engine exerts a force on the TRACK, which causes it to accelerate. Since the engine is connected to the cars, it must accelerate them as well, so the mass being accelerated is engine + cars.
                  $endgroup$
                  – jamesqf
                  10 hours ago










                • $begingroup$
                  @jamesqf The engine also exerts a force on the tracks due to gravity. Since we were just given total force exerted by the engine on the tracks, don't forget to subtract the gravity portion when calculating the force accelerating the train.
                  $endgroup$
                  – Matt
                  9 hours ago










                • $begingroup$
                  @Matt As long as we're being pedantic, they don't say which planet this train is on, or if it's outer space, so the gravitational force is undefined and the question is unsolvable.
                  $endgroup$
                  – Nuclear Wang
                  7 hours ago










                • $begingroup$
                  @NuclearWang Good point! But I disagree that accounting for gravity is pedantic. (Anyway, assuming earth is a pretty standard assumption for entry level physics) The question doesnt specify what the 40kN is exerted on. It could reasonably be on the first wagon in the line, or it could be the tracks. But in fact neither of these are what the "correct" answer assumes. I dont think its fair to expect arbitrary assumptions while also expecting the OP to ignore a very real, very important, force. Especially since the engine apparently exerts a net 40 kN on presisely zero objects.
                  $endgroup$
                  – Matt
                  7 hours ago








                5




                5




                $begingroup$
                "The force is applied on whole system." That's NOT what the words of the problem say. The engine exerts a force---that must be on something else. In this type of analysis, things don't have self-forces. It's a terrible question.
                $endgroup$
                – Bill N
                13 hours ago




                $begingroup$
                "The force is applied on whole system." That's NOT what the words of the problem say. The engine exerts a force---that must be on something else. In this type of analysis, things don't have self-forces. It's a terrible question.
                $endgroup$
                – Bill N
                13 hours ago












                $begingroup$
                @Bill N: The engine exerts a force on the TRACK, which causes it to accelerate. Since the engine is connected to the cars, it must accelerate them as well, so the mass being accelerated is engine + cars.
                $endgroup$
                – jamesqf
                10 hours ago




                $begingroup$
                @Bill N: The engine exerts a force on the TRACK, which causes it to accelerate. Since the engine is connected to the cars, it must accelerate them as well, so the mass being accelerated is engine + cars.
                $endgroup$
                – jamesqf
                10 hours ago












                $begingroup$
                @jamesqf The engine also exerts a force on the tracks due to gravity. Since we were just given total force exerted by the engine on the tracks, don't forget to subtract the gravity portion when calculating the force accelerating the train.
                $endgroup$
                – Matt
                9 hours ago




                $begingroup$
                @jamesqf The engine also exerts a force on the tracks due to gravity. Since we were just given total force exerted by the engine on the tracks, don't forget to subtract the gravity portion when calculating the force accelerating the train.
                $endgroup$
                – Matt
                9 hours ago












                $begingroup$
                @Matt As long as we're being pedantic, they don't say which planet this train is on, or if it's outer space, so the gravitational force is undefined and the question is unsolvable.
                $endgroup$
                – Nuclear Wang
                7 hours ago




                $begingroup$
                @Matt As long as we're being pedantic, they don't say which planet this train is on, or if it's outer space, so the gravitational force is undefined and the question is unsolvable.
                $endgroup$
                – Nuclear Wang
                7 hours ago












                $begingroup$
                @NuclearWang Good point! But I disagree that accounting for gravity is pedantic. (Anyway, assuming earth is a pretty standard assumption for entry level physics) The question doesnt specify what the 40kN is exerted on. It could reasonably be on the first wagon in the line, or it could be the tracks. But in fact neither of these are what the "correct" answer assumes. I dont think its fair to expect arbitrary assumptions while also expecting the OP to ignore a very real, very important, force. Especially since the engine apparently exerts a net 40 kN on presisely zero objects.
                $endgroup$
                – Matt
                7 hours ago




                $begingroup$
                @NuclearWang Good point! But I disagree that accounting for gravity is pedantic. (Anyway, assuming earth is a pretty standard assumption for entry level physics) The question doesnt specify what the 40kN is exerted on. It could reasonably be on the first wagon in the line, or it could be the tracks. But in fact neither of these are what the "correct" answer assumes. I dont think its fair to expect arbitrary assumptions while also expecting the OP to ignore a very real, very important, force. Especially since the engine apparently exerts a net 40 kN on presisely zero objects.
                $endgroup$
                – Matt
                7 hours ago











                3












                $begingroup$

                Imagine if you only had the engine by itself exerting the same 40,000N of thrust, to find the acceleration of the engine you would not do 40000/0kg would you? The 40,000N the engine exert is also used to thrust itself forward.



                But let’s say it didn’t, and that only the wagons were pulled. Well now you have a stationary engine, pulling 5 wagons, each with an acceleration of 4m/s^2. These wagons will catch up to the engine in no time and obliterate the engine from behind. we can at least agree this is not how train works






                share|cite|improve this answer








                New contributor




                Ubaid Hassan is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
                Check out our Code of Conduct.






                $endgroup$













                • $begingroup$
                  If the engine is by itself,, what is exerting thrust on? It can't exert thrust on itself. There must be something else exerting thrust on the engine. Regarding your second paragraph, there is obviously some force acting on the engine, but we aren't told what it is. All we know is that the engine exerts a force of 40000 N, presumably on the train of wagons.
                  $endgroup$
                  – Bill N
                  13 hours ago






                • 1




                  $begingroup$
                  It's tricky but it says "the engine exerts a force" it does not say " the engine exerts a force on the cars". And that's the reason you include the engine! The problem is solvable for either situation.
                  $endgroup$
                  – PhysicsDave
                  13 hours ago










                • $begingroup$
                  I do agree with Physics Dave. Though the very existence of this Q and some comments tell that the exercise could have been written more clearly.
                  $endgroup$
                  – Alchimista
                  11 hours ago










                • $begingroup$
                  @BillN on the rails, naturally.
                  $endgroup$
                  – hobbs
                  10 hours ago


















                3












                $begingroup$

                Imagine if you only had the engine by itself exerting the same 40,000N of thrust, to find the acceleration of the engine you would not do 40000/0kg would you? The 40,000N the engine exert is also used to thrust itself forward.



                But let’s say it didn’t, and that only the wagons were pulled. Well now you have a stationary engine, pulling 5 wagons, each with an acceleration of 4m/s^2. These wagons will catch up to the engine in no time and obliterate the engine from behind. we can at least agree this is not how train works






                share|cite|improve this answer








                New contributor




                Ubaid Hassan is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
                Check out our Code of Conduct.






                $endgroup$













                • $begingroup$
                  If the engine is by itself,, what is exerting thrust on? It can't exert thrust on itself. There must be something else exerting thrust on the engine. Regarding your second paragraph, there is obviously some force acting on the engine, but we aren't told what it is. All we know is that the engine exerts a force of 40000 N, presumably on the train of wagons.
                  $endgroup$
                  – Bill N
                  13 hours ago






                • 1




                  $begingroup$
                  It's tricky but it says "the engine exerts a force" it does not say " the engine exerts a force on the cars". And that's the reason you include the engine! The problem is solvable for either situation.
                  $endgroup$
                  – PhysicsDave
                  13 hours ago










                • $begingroup$
                  I do agree with Physics Dave. Though the very existence of this Q and some comments tell that the exercise could have been written more clearly.
                  $endgroup$
                  – Alchimista
                  11 hours ago










                • $begingroup$
                  @BillN on the rails, naturally.
                  $endgroup$
                  – hobbs
                  10 hours ago
















                3












                3








                3





                $begingroup$

                Imagine if you only had the engine by itself exerting the same 40,000N of thrust, to find the acceleration of the engine you would not do 40000/0kg would you? The 40,000N the engine exert is also used to thrust itself forward.



                But let’s say it didn’t, and that only the wagons were pulled. Well now you have a stationary engine, pulling 5 wagons, each with an acceleration of 4m/s^2. These wagons will catch up to the engine in no time and obliterate the engine from behind. we can at least agree this is not how train works






                share|cite|improve this answer








                New contributor




                Ubaid Hassan is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
                Check out our Code of Conduct.






                $endgroup$



                Imagine if you only had the engine by itself exerting the same 40,000N of thrust, to find the acceleration of the engine you would not do 40000/0kg would you? The 40,000N the engine exert is also used to thrust itself forward.



                But let’s say it didn’t, and that only the wagons were pulled. Well now you have a stationary engine, pulling 5 wagons, each with an acceleration of 4m/s^2. These wagons will catch up to the engine in no time and obliterate the engine from behind. we can at least agree this is not how train works







                share|cite|improve this answer








                New contributor




                Ubaid Hassan is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
                Check out our Code of Conduct.









                share|cite|improve this answer



                share|cite|improve this answer






                New contributor




                Ubaid Hassan is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
                Check out our Code of Conduct.









                answered 14 hours ago









                Ubaid HassanUbaid Hassan

                1419




                1419




                New contributor




                Ubaid Hassan is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
                Check out our Code of Conduct.





                New contributor





                Ubaid Hassan is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
                Check out our Code of Conduct.






                Ubaid Hassan is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
                Check out our Code of Conduct.












                • $begingroup$
                  If the engine is by itself,, what is exerting thrust on? It can't exert thrust on itself. There must be something else exerting thrust on the engine. Regarding your second paragraph, there is obviously some force acting on the engine, but we aren't told what it is. All we know is that the engine exerts a force of 40000 N, presumably on the train of wagons.
                  $endgroup$
                  – Bill N
                  13 hours ago






                • 1




                  $begingroup$
                  It's tricky but it says "the engine exerts a force" it does not say " the engine exerts a force on the cars". And that's the reason you include the engine! The problem is solvable for either situation.
                  $endgroup$
                  – PhysicsDave
                  13 hours ago










                • $begingroup$
                  I do agree with Physics Dave. Though the very existence of this Q and some comments tell that the exercise could have been written more clearly.
                  $endgroup$
                  – Alchimista
                  11 hours ago










                • $begingroup$
                  @BillN on the rails, naturally.
                  $endgroup$
                  – hobbs
                  10 hours ago




















                • $begingroup$
                  If the engine is by itself,, what is exerting thrust on? It can't exert thrust on itself. There must be something else exerting thrust on the engine. Regarding your second paragraph, there is obviously some force acting on the engine, but we aren't told what it is. All we know is that the engine exerts a force of 40000 N, presumably on the train of wagons.
                  $endgroup$
                  – Bill N
                  13 hours ago






                • 1




                  $begingroup$
                  It's tricky but it says "the engine exerts a force" it does not say " the engine exerts a force on the cars". And that's the reason you include the engine! The problem is solvable for either situation.
                  $endgroup$
                  – PhysicsDave
                  13 hours ago










                • $begingroup$
                  I do agree with Physics Dave. Though the very existence of this Q and some comments tell that the exercise could have been written more clearly.
                  $endgroup$
                  – Alchimista
                  11 hours ago










                • $begingroup$
                  @BillN on the rails, naturally.
                  $endgroup$
                  – hobbs
                  10 hours ago


















                $begingroup$
                If the engine is by itself,, what is exerting thrust on? It can't exert thrust on itself. There must be something else exerting thrust on the engine. Regarding your second paragraph, there is obviously some force acting on the engine, but we aren't told what it is. All we know is that the engine exerts a force of 40000 N, presumably on the train of wagons.
                $endgroup$
                – Bill N
                13 hours ago




                $begingroup$
                If the engine is by itself,, what is exerting thrust on? It can't exert thrust on itself. There must be something else exerting thrust on the engine. Regarding your second paragraph, there is obviously some force acting on the engine, but we aren't told what it is. All we know is that the engine exerts a force of 40000 N, presumably on the train of wagons.
                $endgroup$
                – Bill N
                13 hours ago




                1




                1




                $begingroup$
                It's tricky but it says "the engine exerts a force" it does not say " the engine exerts a force on the cars". And that's the reason you include the engine! The problem is solvable for either situation.
                $endgroup$
                – PhysicsDave
                13 hours ago




                $begingroup$
                It's tricky but it says "the engine exerts a force" it does not say " the engine exerts a force on the cars". And that's the reason you include the engine! The problem is solvable for either situation.
                $endgroup$
                – PhysicsDave
                13 hours ago












                $begingroup$
                I do agree with Physics Dave. Though the very existence of this Q and some comments tell that the exercise could have been written more clearly.
                $endgroup$
                – Alchimista
                11 hours ago




                $begingroup$
                I do agree with Physics Dave. Though the very existence of this Q and some comments tell that the exercise could have been written more clearly.
                $endgroup$
                – Alchimista
                11 hours ago












                $begingroup$
                @BillN on the rails, naturally.
                $endgroup$
                – hobbs
                10 hours ago






                $begingroup$
                @BillN on the rails, naturally.
                $endgroup$
                – hobbs
                10 hours ago













                1












                $begingroup$

                The question is flawed in that can be deemed to be ambiguous and therefore requires interpretation.



                Using Newton’s second law requires the system under consideration to be defined and that has not been done in this question.



                Without the system being defined the statement




                the engine exerts a force of 40000N




                is the cause of the ambiguity.



                So which is it?



                1 The engine is exerting an external force of 40000N on the system of five wagons.



                2 The engine exerts an internal force of 40000N on the system of five wagons and the engine.



                3 There is an external force of 40000N on the engine and five wagons.



                The author of the book really meant option 3 but given the way the question was written I have to agree with the OP that an external force of 40000N is acting on the five wagons whcich is option 1.






                share|cite|improve this answer











                $endgroup$


















                  1












                  $begingroup$

                  The question is flawed in that can be deemed to be ambiguous and therefore requires interpretation.



                  Using Newton’s second law requires the system under consideration to be defined and that has not been done in this question.



                  Without the system being defined the statement




                  the engine exerts a force of 40000N




                  is the cause of the ambiguity.



                  So which is it?



                  1 The engine is exerting an external force of 40000N on the system of five wagons.



                  2 The engine exerts an internal force of 40000N on the system of five wagons and the engine.



                  3 There is an external force of 40000N on the engine and five wagons.



                  The author of the book really meant option 3 but given the way the question was written I have to agree with the OP that an external force of 40000N is acting on the five wagons whcich is option 1.






                  share|cite|improve this answer











                  $endgroup$
















                    1












                    1








                    1





                    $begingroup$

                    The question is flawed in that can be deemed to be ambiguous and therefore requires interpretation.



                    Using Newton’s second law requires the system under consideration to be defined and that has not been done in this question.



                    Without the system being defined the statement




                    the engine exerts a force of 40000N




                    is the cause of the ambiguity.



                    So which is it?



                    1 The engine is exerting an external force of 40000N on the system of five wagons.



                    2 The engine exerts an internal force of 40000N on the system of five wagons and the engine.



                    3 There is an external force of 40000N on the engine and five wagons.



                    The author of the book really meant option 3 but given the way the question was written I have to agree with the OP that an external force of 40000N is acting on the five wagons whcich is option 1.






                    share|cite|improve this answer











                    $endgroup$



                    The question is flawed in that can be deemed to be ambiguous and therefore requires interpretation.



                    Using Newton’s second law requires the system under consideration to be defined and that has not been done in this question.



                    Without the system being defined the statement




                    the engine exerts a force of 40000N




                    is the cause of the ambiguity.



                    So which is it?



                    1 The engine is exerting an external force of 40000N on the system of five wagons.



                    2 The engine exerts an internal force of 40000N on the system of five wagons and the engine.



                    3 There is an external force of 40000N on the engine and five wagons.



                    The author of the book really meant option 3 but given the way the question was written I have to agree with the OP that an external force of 40000N is acting on the five wagons whcich is option 1.







                    share|cite|improve this answer














                    share|cite|improve this answer



                    share|cite|improve this answer








                    edited 10 hours ago

























                    answered 13 hours ago









                    FarcherFarcher

                    52.1k340110




                    52.1k340110























                        0












                        $begingroup$

                        The force of 40 kN is exerted on the rails, not the wagons



                        We have to assume that the engine and wagons are all coupled together, and therefore that they move together. If the engine exerted a force on the wagons but did not move with them, the wagons would provide a different force in reaction. If the train was off the track, it would not be exerting that force on the wagons.



                        The engine turns the wheels which exert a force backwards on the rails, which also imparts the reaction forwards on the train.



                        That force is transmitted from the rails through the engine and the coupling to the wagons.



                        To take a different circumstance, suppose the train was going uphill? The engine will still exert a force of the same magnitude, but this time the balance of forces and angles are different, so the acceleration is different too.



                        Thus that force must accelerate the total mass of the engine and the wagons together.






                        share|cite|improve this answer









                        $endgroup$


















                          0












                          $begingroup$

                          The force of 40 kN is exerted on the rails, not the wagons



                          We have to assume that the engine and wagons are all coupled together, and therefore that they move together. If the engine exerted a force on the wagons but did not move with them, the wagons would provide a different force in reaction. If the train was off the track, it would not be exerting that force on the wagons.



                          The engine turns the wheels which exert a force backwards on the rails, which also imparts the reaction forwards on the train.



                          That force is transmitted from the rails through the engine and the coupling to the wagons.



                          To take a different circumstance, suppose the train was going uphill? The engine will still exert a force of the same magnitude, but this time the balance of forces and angles are different, so the acceleration is different too.



                          Thus that force must accelerate the total mass of the engine and the wagons together.






                          share|cite|improve this answer









                          $endgroup$
















                            0












                            0








                            0





                            $begingroup$

                            The force of 40 kN is exerted on the rails, not the wagons



                            We have to assume that the engine and wagons are all coupled together, and therefore that they move together. If the engine exerted a force on the wagons but did not move with them, the wagons would provide a different force in reaction. If the train was off the track, it would not be exerting that force on the wagons.



                            The engine turns the wheels which exert a force backwards on the rails, which also imparts the reaction forwards on the train.



                            That force is transmitted from the rails through the engine and the coupling to the wagons.



                            To take a different circumstance, suppose the train was going uphill? The engine will still exert a force of the same magnitude, but this time the balance of forces and angles are different, so the acceleration is different too.



                            Thus that force must accelerate the total mass of the engine and the wagons together.






                            share|cite|improve this answer









                            $endgroup$



                            The force of 40 kN is exerted on the rails, not the wagons



                            We have to assume that the engine and wagons are all coupled together, and therefore that they move together. If the engine exerted a force on the wagons but did not move with them, the wagons would provide a different force in reaction. If the train was off the track, it would not be exerting that force on the wagons.



                            The engine turns the wheels which exert a force backwards on the rails, which also imparts the reaction forwards on the train.



                            That force is transmitted from the rails through the engine and the coupling to the wagons.



                            To take a different circumstance, suppose the train was going uphill? The engine will still exert a force of the same magnitude, but this time the balance of forces and angles are different, so the acceleration is different too.



                            Thus that force must accelerate the total mass of the engine and the wagons together.







                            share|cite|improve this answer












                            share|cite|improve this answer



                            share|cite|improve this answer










                            answered 10 hours ago









                            Phil HPhil H

                            870413




                            870413






























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