Physics Help?!?

Question

If a person is in a car starts driving at the constant speed V. If a sled is at rest at the top of a slope, it has some potential energy. Once it rolls down the hill, it has kinetic energy, while moving at the same constant speed V. , how is that from the reference frame of the car, Kinetic Energy = 0, thus, ME initial is not = to ME final, violating newton's law.

Answer 1

You converted reference frames.

Here's a way to think about the problem:
If you are inside a car moving at speed V and you blow a feather of mass m forward at a speed v with respect to you, what is the energy of the feather?
With respect to you, it is .5*m*v^2
with respect to the road, it is
.5*m*(V+v)^2
where did the additional energy come from when you switch to the reference frame of the road?
It came from the fact that the feather was inside the car and got accelerated to speed V before you added the bit more energy to get it going V+v. But, since you were also accelerated to speed V, the energy the feather required to go the incremental speed v, was .5*m*v^2, w/r/t you.

This concept becomes critical when you start working in relativistic physics where there is the constant C for the speed of light in all reference frames.

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