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A work-energy situation



I am reposting my problem after making the mass of the plate "much larger"
than the mass of the sliding cube. Without this you would say that I am
describing an impossible situation (the plate of small mass can not be at
rest while slowing down the cube).


An iron cube whose mass is m is small slides horizontally over a long iron
plate. The mass of the plate M is much larger than m. The plate is a rest
while the cube has an initial kinetic energy K. Friction brings the cube
to rest before its base reaches the plate's boundary. Everything is in the
vacuum and the internal energy, dU, added to both pieces of iron is not
radiated. [In principle, dU can be calculated from the observed change of
temperature but no thermometer is available.] Which of the following is
correct and why?

1. The internal energy change, dU, is equal to K.
2. Only part of K goes into dU, the rest is used to do work W=R*x, where
R is the force of kinetic friction and x is the distance the cube
slides before comming to rest.
3. Neither of the above is correct.

I am really confused in this situation. My frame of reference (for K and x)
is attached to the plate.
Make sure I understand you.
Ludwik Kowalski