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[Phys-L] Re: Momentum Again



At 02:13 PM 12/7/2005, Dr. George, you wrote:
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.
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Dr. George Spagna

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.

As it is quite plain that the concept of kinetic energy is in fact extremely
useful, we should simply note that KE is referable to some point.
That is to say, there enters here the concept of Galiliean relativity,
and the idea of usable energy. And yes, the implication is about the
transfer of energy from object to object and to environment.
Putting a box around the arena of interest is not a convincing
solution to the pathological features of an upper bound to energy loss
from lossy collisions while maintaining momentum.

Moreover, there is another series of collisions that lose more than half the
initial kinetic energy of course. These are the ones where not only is
kinetic energy not contained in the objects of interest, but neither is
the momentum.



Brian Whatcott Altus OK Eureka!
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