Re: [Phys-l] Collision of irregular bodies

[John Denker <jsd@av8n.com>]

Bob LaMontagne wrote:

> In the usual "plane of contact" collision, the force is perpendicular to the
> plane. 

Really?  How do you know?  What's the measure for "usual"?

Has anybody done an  experiment  that shows the force is
reproducibly perpendicular to the plane of contact?

In every experiment I can think of, there are readily-observable
force components in the plane of contact.  Previously mentioned
examples include
 -- billiard balls(*)
 -- dropping a broomstick onto the floor

Another example that comes immediately to mind is table tennis
(ping pong).  I've seen 10-year-olds try to play the game using
only normal forces ... but I cannot consider that the "usual"
case.  The first kid who figures out how to use the non-normal
forces will win every time.

Are these not valid observations?  Are they not representative
of the "usual" case???






==================

(*) The experiment described in Problem 1.6 on page 10 of Ron
Shepard's treatise is a particularly clean example of the point
I'm making.  It easily establishes a nonzero lower bound on the
coefficient of friction.  (I would quibble with the wording of
the problem, however;  this does not measure "the" coefficient
of friction, only a lower bound.  Obtaining a _tight_ lower
bound requires more effort.)
  http://www.tcbilliards.com/articles/physics.html