# Binding Energies in Different Frames

Q:
My Doubt is while calculating KE on earth we consider object's relative velocity with respect to earth although earth is moving with a speed, also sun is moving, also galaxy is moving.My point is that we use this KE in calculting Binding Energies also so what i am missing?
- Kaushik (age 18)
Nagpur, Maharshtra, India
A:

Nice question. Say you have two particles bound together. In any classical frame you can write their kinetic energy as the kinetic energy of the net mass moving at the center-of-mass velocity plus the kinetic energy of the motions relative to the center of mass. (the cross terms cancel.) Since conservation of momentum holds, the center-of-mass motion kinetic energy won't change. The net change in this frame is then just the same as the change in the center-of-mass frame. So we can talk about binding energies, etc., wthout worrying about which frame is used.

Mike W.

(published on 07/31/2020)

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