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Re: [Phys-l] momentum and energy



Oh.

I thought you objected to my criticism of Rob's suggestion that his collision w/ a wall was inelastic because it "reflects" w/ v. sl. less KE.
"... it must recoil with a slightly lower speed (therefore, it is slightly inelastic and ..."

I think the assumption is(was) the wall and ball are elastic.

bc, who chooses to treat elastic as one should treat unique (no comparative; are absolute).

Jack Uretsky wrote:

Total.
Regards,
Jack

On Mon, 13 Nov 2006, Bernard Cleyet wrote:


Jack!

Do you mean the KE of the incident body or the total. i.e. no "dissipation"?

I was taught the latter, but that was sl. over 50 years ago.

bc

Jack Uretsky wrote:


I suggest that the definition of "elastic collision" be that kinetic
energy is conserved in the collision. This definition agrees with
accepted practice in every frame, classicly and quantum mechanically.
Regards,
Jack

On Mon, 13 Nov 2006, Bernard Cleyet wrote:




Upon re-reading I discover I hadn't read carefully. My yes referred to
the first part of Rob's very long sentence. However, the second part,
if i remember correctly is wrong. The collision is elastic despite the
sl. loss in the ball's KE, whatever the frame.

http://en.wikipedia.org/wiki/Elastic_collision

However, I think only in the atomic scale are collisions possibly elastic.

bc


Bernard Cleyet wrote:




interleaved:

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...)







in a finite U. yes. Of course the diff. is not measurable -- because of
noise i.e. the limitation is not just technological.






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)







Again in a finite U. it must be non zero. But measurement is not just
technologically impossible, but theoretically (scientifically)
impossible, however, it is not logically impossible.

bc, breathlessly awaiting jsd's response. OTOH, maybe not!






Regards,
Rob Spencer



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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_______________________________________________
Forum for Physics Educators
Phys-l@carnot.physics.buffalo.edu
https://carnot.physics.buffalo.edu/mailman/listinfo/phys-l







_______________________________________________
Forum for Physics Educators
Phys-l@carnot.physics.buffalo.edu
https://carnot.physics.buffalo.edu/mailman/listinfo/phys-l






_______________________________________________
Forum for Physics Educators
Phys-l@carnot.physics.buffalo.edu
https://carnot.physics.buffalo.edu/mailman/listinfo/phys-l