Re: energy change

[John Mallinckrodt <ajmallinckro@CSUPOMONA.EDU>]

On Wed, 6 Feb 2002, John Barrer wrote:

> In a collision, the ball transfers energy into the wall on the
> "way in"  and recovers it (all of it if perfectly elastic) on
> the "way out".

Perhaps *some* of the energy is transferred to and from the wall,
but certainly no amount worth taking into account unless the wall
is uncommonly "soft."

> I think it's misleading to claim there is no energy transfer
> in this process.

On the contrary; what is misleading is to suggest that there IS
any substantial energy transfer in the most likely version of this
process.  I conjure up a picture of a deformable and elastic ball
hitting a very hard wall at a speed that will not destroy either
the ball or the wall.  In that case there is virtually no energy
transfer to or from the wall. The kinetic energy that the ball had
is momentarily converted to and stored IN THE BALL as elastic
potential energy and is then converted back to kinetic energy as
the ball rebounds.

> If you deposit your paycheck and immediately thereafter
> withdraw a like amount, would you say your account had no
> activity? John BArrere

Of course not; but the analogy is faulty.

If you enter a mall holding a hundred dollar bill in your hand,
stick it in your pocket for security while window shopiing, and
then remove it again when you leave, would you say that you had
transfered money to and from the mall?

John Mallinckrodt      mailto:ajm@csupomona.edu
Cal Poly Pomona          http://www.csupomona.edu/~ajm