Re: [Phys-l] momentum and energy

[Jack Uretsky <jlu@hep.anl.gov>]

On Mon, 13 Nov 2006, Spencer, Rob wrote:

> Hi,
> Follow up statements to make sure my understanding is clear (thanks
> for the replies)...T or F ... grade my statements...
> a)  theoretically, it is impossible for the ball to recoil with "exactly"
> the same speed as its initial speed (assuming no change in mass)...it 
> must recoil with a slightly lower speed (therefore, it is slightly 
> inelastic and in the CM frame, we would not see the velocity just 
> flipped 
> as with a perfectly elastic collision...Dr. Denker, go easy on me here 
> since this is in opposition to your claim below...)

	Looking for "slight" effects evades the essential physics of the
	problem.  As physicists we learn to deal with idealizations that
	gives us the big picture.  The big picture has already been
	wrapped up in the concise comment:
			K.E> = p^2/2M
	which I would summarize by saying that an infinite mass can absorb
	arbitrarily large momentum without picking up any kinetic energy.

> b)  The wall (earth) does recoil with a non-zero momentum...the velocity
	being ridiculously small...the bigger the mass ratio of wall to
	ball, the smaller the velocity ratio of wall to ball...but
	implying that there really is an associated nonzero K
	for the wall (earth)


> 	On a more positive note, the notion of tiny deflections can be
	useful in solving statics problems.  It is called "the principle
	of virtual work", discussedin recent postings.

> Regards,
> Rob Spencer
						Regards,
							Jack>
> The trick is here:
> > recoils with a velocity equal in magnitude to its initial velocity.
>
> That's strictly true in the CM frame and approximately /but not exactly/
> true in the lab frame.
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