Re: why pseudowork (NOT)

[John Mallinckrodt <ajmallinckro@CSUPOMONA.EDU>]

On Fri, 29 Oct 1999, Ludwik Kowalski wrote:

> OK,  how would you accomplish this "trick" with an air track?
> I give you the 1 kg glider and the 2 kg glider. Also devices to
> impose initial speeds of 1 m/s on each glider. They move
> toward each other. The task is to make sure the 2 kg is at rest
> after the collision. We know the tricks for producing inelastic
> collisions (velcro, sp?), we know the tricks for elastic collisions
> (good springs). What is your "trick" to stop the 2 kg mass and
> to conserve the linear momentum?

Ludwik,

As Bob and brian have said in their own ways, momentum will conserve
itself without any help from us. To stop the 2 kg glider, we simply need
to make the surfaces properly inelastic.  Velcro will make the collision
*totally* inelastic, good springs will make it almost elastic.  Let's just
remove the springs and treat the front surfaces of the gliders with balsa
wood attachments or something like that until the coefficient of
restitution is about 1/2.  Yes, it will take a bit of trial and error to
get it right, but surely you can see that this is merely a technical
detail.

Since you have Interactive Physics you could also simulate the collision.
Just make the "elasticity" of both objects 0.5.

This morning when I posted this question, I really had no intention of
getting sidetracked onto the not very interesting topic of momentum
conservation in one dimensional collisions.  I was far more interested in
how people viewed the work-energy relationships in this very ordinary
collision.  I am sorry that that more revealing question has been lost in
the noise.

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