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Re: [Phys-l] momentum and energy



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