From: phys-l-bounces@carnot.physics.buffalo.edu on behalf of Bob LaMontagne
Sent: Tue 10/17/2006 5:54 PM
To: 'Forum for Physics Educators'
Subject: Re: [Phys-l] conservation versus constancy
I don't follow the claim about the work being zero in one frame implies it
is zero in all others. In a collision between identical steel balls (one
moving and the other at rest in the lab frame), no work is done in the CM
system because there is no displacement of the contact point during the
collision. In the lab frame, there is a displacement of the contact point,
resulting in positive work being done on one ball with the same amount of
negative work being done on the other. One ball slows down and stops, the
other goes from rest to the original speed of the other. The NET work may be
zero but the individual works are done on different bodies, so the effects
are different. This is a primitive slingshot effect. A better one is to have
the incoming ball to be many times more massive. The lighter one picks up
twice the speed of the original due to the work done on it in the lab frame.
A positively charged, fast moving, heavy sphere hitting a positively charged
light one at rest produces exactly the same effect - even without direct
surface contact. That is easy to see. So why should the attractive case be
so different?