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Re: [Phys-l] Collision of irregular bodies

----- Original Message ----- From: "Brian Whatcott" <betwys1@sbcglobal.net>
To: "Forum for Physics Educators" <phys-l@carnot.physics.buffalo.edu>
Sent: Saturday, August 12, 2006 12:14 PM
Subject: Re: [Phys-l] Collision of irregular bodies


At 09:23 AM 8/12/2006, R McDermott, you wrote:

///
>>Agreed. Now how about for a single point of contact? Can the mass
>>distribution, etc affect the direction of force...?
>
> Yes. Would you care for imaginary examples?

Yes, please. I'm having difficulty seeing how that (forces NOT normal to
the surfaces, and in the absence of tangential forces) could happen.

Hmmm. This requirement is a little too restrictive for me.

If two bodies collide at a point, AND
if there are no side forces at that point,
THEN
there are only normal forces at that point of contact.
(That was the point to which you took exception I think.)

Well, it wasn't so much that I took exception, I just couldn't envision how that could happen without some tangential force, and in the absence of friction, I couldn't see where one would come from or how mass distribution could impact on the direction of the forces at the point of contact. I was operating under the assumption that I was missing something and simply looking for clarification.

I wanted to describe, for example, two rigid beams moving
freely in space, each rotating and each
provided with hard round balls, one at each end of each beam.
You can imagine several varieties of interesting results of
contra-rotation, and oblique touching at the end balls.
Specifically, the osculatory plane can have a variety of
orientations, and with the variability of the imaginary centers
of mass of these objects, the normal force of contact
could provide linear or angular acceleration of each object.

Absolutely. The effects in such a situation would have infinite variations depending on orientation and initial boundary conditions, as well compound effects if there were more than one point of contact. Similarly, deformation would be likely to change the direction of the forces at the point of contact over time. And, of course, if there were any forces of adhesion, friction, etc, then the force pair would not have to be normal to the surfaces. That's all obvious. Ok... Thanks, Brian.