Re: [Phys-l] Collision of irregular bodies

["Bob LaMontagne" <rlamont@postoffice.providence.edu>]

????? The thread had long ago turned to frictionless examples. My apologies
for not reinventing the wheel in my post.

The real point was to expand on the example brought up by Brian that
indicated that even for frictionless surfaces, there are specific examples
that can be contrived where the simple prescription of contact forces
perpendicular to a plane of contact are inadequate.

Bob at PC

> -----Original Message-----
> From: phys-l-bounces@carnot.physics.buffalo.edu [mailto:phys-l-
> bounces@carnot.physics.buffalo.edu] On Behalf Of John Denker
> Sent: Monday, August 14, 2006 1:19 PM
> To: Forum for Physics Educators
> Subject: Re: [Phys-l] Collision of irregular bodies
>
> 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
>
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