A locomotive pulls a series of wagons. Which is the correct analysis of the situation?

A. because action always equals the reaction,the locomotive cannot pull the wagons,-the wagons pull backward just as hard as the locomotive pulls forward, so there is no motion.

B. The train moves forward because the locomotive pulls forward slightly harder on the wagons than the wagons pull backward on the locomotive

C. the locomotive gets the wagons to move by giving them a tug during which the force on the wagons is momentarily greater than the force exerted by the wagons on the locomotive

D. the locomotive can pull the wagons forward only if it weighs more than the wagons

E. The locomotive's force on the wagons is as strong as the force of the wagons on the locomotive,but the frictional force on the locomotive is forward and large while the backward fricitonal force on the wagons is small.

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asked by alexa
  1. Very poor question.
    a. would be true if the word "accelerating" were inserted between "no motion". As it is, it is false
    b. if Locomotive pulls means net force of the locomotive, it is false, unless the train is accelerating.
    c. Again, true during the moment of acceleration.
    d. nonsense
    e. nonsense.

    So the question is what does the question mean "a locomotive pulls a series of wagons" This is very indefinite. The central question is the train accelerating, constant motion (including stopped)?

    I suspect Your instructor mean c to be the only right answer, but instructors get paid to do better than this.

    If net force is greater than retarding force, acceleration happens. If net force is equal to retarding force, acceleration is zero, and the system remains in whatever motion it was in.

  2. E is the correct answer. This question was written by a leading physics education researcher (I'm reading his book right now, in which he explains why the answer is E.)

    I can see how it would be easier if he said "accelerating," but questions like this are intended to be thought provoking, which is often better achieved by using less technical language.

    Bobpursley's explanation of "a" is incorrect. If he had written "accelerating" in there, E is still the only one that is correct. Bob's explanation would tell us that no train can ever accelerate, period. "A" could only be correct if he had specifically said the train is NOT accelerating.

    I will assume Bob means "net force ON the locomotive" since "net force OF..." doesn't make sense. Objects exert individual forces, they feel net forces. If it said "net force on the locomotive" in B, that answer would be essentially the same as E -- which is the correct answer, not a false one!

    D is clearly false. Weight is gravity pulling down, not train parts pulling forward or back. It's not relevant to the question. In other places, Bob seems to think that the question should have answer choices that are clearly true or clearly false. Here, he takes one that is clearly false and derides it as "nonsense."

    The third law requires that the force of the locomotive on the wagons be equal and opposite to the force of the wagons on the locomotive. If forward acceleration ever occurs, then there must be a net forward force on both locomotive, and wagons. With the wagon's, that's easy to explain. The locomotive pulls them forward. With the locomotive, if the wagons are pulling back, what is pushing it forward? It's the interaction with the tracks -- which must be stronger than the backward pull of the wagons.

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    posted by George

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