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[Phys-L] Re: Momentum Again



At 08:29 AM 12/8/2005, Joel Rauber, you wrote:

I don't really understand the following:

| [Brian]
| If we start from the reference of the center of mass of the
| planet Earth, then there is no kinetic energy on Earth.
|
| Or if it were possible to assign a center of mass of the Universe,
| there would be no kinetic energy in the universe.
|
[Joel]
The calculations that I am familiar with say that the kinetic energy of
a system is the KE due to the c-of-mass motion plus the KE of the bits
and pieces relative to the c-of-mass. So in the c-of-mass frame one
still has a non-zero portion; namely the KE of the bits and pieces
relative to the c-of-mass.

(Unless I misunderstood the intent of the above quote, which is quite
possible.)


I had this contribution of George's in mind:
[George]
"This point has probably been made during this thread, but let me toss it
in again:

The question "Why does 1/2 the kinetic energy 'disappear' in a
completely inelastic collision?" is frame dependent, along with the
assignment of a particular value for the kinetic energy of the system.
Viewed from the center of mass, all of the kinetic energy vanishes."
//

Dr. George Spagna



Brian Whatcott Altus OK Eureka!
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