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Re: Work/Energy theorem ?



Some belated thoughts on "Charles" question on where the kinetic energy
of a tree goes when seen from a moving train vs. after the train stops.

1. It must be emphasized to students that BOTH kinetic energy and the work
done by forces are dependent of the frame of reference chosen - even if
both are inertial frames. The KE of an object changes for the obvious
reason, V' = V + Vrel, but the work done by forces changes because the distance
traveled during any time inerval is also different, F X' = F (X + Vrel t)

2. Another approach would be to consider the tree in isolation and the view
of it from the reference frame of the train as it stops. The tree then is
seen to accelerate, and one could interpret it as losing KE because negative
work is being done by the "ficticious" force causing that acceleration.