Re: [Phys-l] Collision of irregular bodies

["Richard Tarara" <rbtarara@sprynet.com>]

To continue down the slippery slope of making this thread totally
irrelevant to the original poster and to the students for which it was
intended---since the forces in question are actually electrical forces
(repulsion between electron clouds that get too close I'm guessing...or do
we have to use QM here?), then we could proceed from there.   Anyway, it
seems to be that if we could simplify to one atom in object A interacting
with one atom in object B then in fact the force vector could be in almost
any orientation relative to the surfaces of the two objects depending on
the orientation of the two atoms.  However, when we consider that for two
macroscopic objects, the number of atoms involved are uncountable and the
orientations between interacting atom pairs (electron clouds?) are totally
random, then there should be a good cancelation of all components in the
plane of the surfaces leaving only the perpendicular components--the normal
force.  

Now of course there are NO perfectly flat surfaces to interact when dealing
with two real objects and friction (which is an extension of the electrical
interactions above) is always present and if the object(s) are spinning
then _maybe_ the forces are not just electrostatic (I haven't a clue here),
and of course the surfaces can deform (more electrical interactions), etc. 

As always, we can make any question, any physical situation, just about as
complex as we want.  ;-)

Rick


> [Original Message]
> From: John Denker <jsd@av8n.com>
> To: Forum for Physics Educators <phys-l@carnot.physics.buffalo.edu>
> Date: 8/14/2006 1:44:26 PM
> 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???