# A good Concept Q. about K.E

• yttuncel
In summary, the conversation discusses the concept of work required to stop a speeding truck from two different frames of reference - one from the perspective of an outside observer and the other from the perspective of the truck driver. The explanation provided involves the idea that in both cases, the truck initially has kinetic energy and loses it, but the perspective of the observer may differ. The conversation also delves into the idea of relative motion and how it affects our perception of events.

## Homework Statement

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.

## 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?

Sounds good. In one case, the truck initially has KE and loses it. In the other, it initially has zero KE and gains some.

Thanks !

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'

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'does Oxford stop at this train'
Neat! Must remember that. But it is hard to think of a relativist saying it when he is experiencing the accelerations of a train.

I know ! that is beyond me !