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We also talk about the forms
objects store energy in. A moving object stores kinetic energy. This is
to show that energy is constantly being shifted around from one form or
body to another. Let me know what you think.
Leon
I've become somewhat leery of the idea on an object "storing" KE. It doesn't seem to me be a property intrinsic to an object at all. Rather it is a property of a system of the object PLUS some external reference system (probably the earth, for ordinary lab experiments). After all, the object's speed must be measured relative to something external. So in this sense of being associated with a system and not an individual object, KE is like PE.
One of my favorite examples which illustrates the frame dependence of KE (and thus its change) is the calculation of the escape speed v of a rocket of mass m=. Let M and R be earth's mass and radius. Calculating first in earth's frame, we have:
initial KE + PE = final KE + PE
0.5mv^2 - GMm/R = 0 + 0
thus v = sqrt(2GM/R)
Next calculate in a frame moving at speed u relative to earth, say in the direction of launch of the rocket:
0.5m(v-u)^2 - GMm/R = 0.5mu^2 + 0
Needless to say, one doesn't find the same solution for v. The error lies in carelessly referring to transforms between KE "of the rocket" and PE "of the rocket," whereas it is the entire rocket-earth system that needs to be considered. -Carl
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