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Re: [Phys-l] momentum and energy



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...)
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)

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.