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Homework Help: A good Concept Q. about K.E

  1. Nov 16, 2011 #1
    1. The problem statement, all variables and given/known data

    A truck speeding down the highway has a lot of kinetic energy relative to a stopped state trooper, but no kinetic energy relative to the truck driver. In these two frames of reference, is the same amount of work required to stop the truck? Explain.

    2. Relevant equations

    3. The attempt at a solution

    I thought like this:

    a) If we are looking from outside, truck has the velocity V and the environment has none. Thus dK=W necessary to stop the truck would be equal to K.E of the truck.

    b) On the other hand, if we are the driver, the environment has the velocity V and we have none. Thus again W=dK necessary to stop the 'environment' would be equal to the K.E of the truck in part a.

    I think it is logical, and all I ask from you is to tell me if the explanation makes sense to you? Is it clear? Does it involve any contradictions or any sign errors?
  2. jcsd
  3. Nov 16, 2011 #2


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    Sounds good. In one case, the truck initially has KE and loses it. In the other, it initially has zero KE and gains some.
  4. Nov 16, 2011 #3
    Thanks !
  5. Nov 16, 2011 #4
    It is a nice question and a good discussion point. It reminds me of a similar situation:You drop a ball and it hits the ground.... Does the ball fall to the ground or does the ground come up to meet the ball? If you live on the ground I suppose your view is that the ball falls to the ground. If you live on the ball..????????
    The forces do not supply the answer (if there is one) because the force on the ball = the force on the Earth (Newton's 3rd law)
    I suppose it all depends on your point of view and when it comes down to it which is the easier view to take. Most of us are standing on the Earth not on separate balls so we do have something in common. I love this sort of physics thinking.
    There is a lovely quote (in England) regarding one of Einstein's associates and the ideas of relativity, He asked the train conductor 'does Oxford stop at this train'
    Last edited: Nov 16, 2011
  6. Nov 16, 2011 #5


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    Neat! Must remember that. But it is hard to think of a relativist saying it when he is experiencing the accelerations of a train.
  7. Nov 16, 2011 #6
    I know !!!! that is beyond me !!
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