Re: [Phys-l] Collision of irregular bodies

[John Denker <jsd@av8n.com>]

R. McDermott wrote:
> Regardless of what it "sounds like" to you, John, have you anything to add 
> that might clarify how it might be otherwise, or is a flip response to be 
> the sum total of your contribution?  You posted that this did not have to be 
> so.  I don't see how that could happen and asked you , nicely, for 
> clarification.  

As I previously stated, the OP specified "smooth", but that is not
necessarily the same as frictionless.  If you want to leap from
smooth to frictionless, that's up to you.

Few things in life are frictionless.  OTOH few things are perfectly
smooth, so obviously some idealizations are involved ... but depending
on how many idealizations you make, the question might be trivial, or
intractable, or anywhere in between.

If you think there is a trivial solution, fine ... but don't have a snit
fit if somebody points out that your solution is trivial.  I usually assume
that when people ask a question, they have some nontrivial reason for
asking.

As another point along the same line, the idea that the bodies could
be "irregular" on one length scale but "smooth" on another length scale
is a sophisticated idea, indeed a tricky idea;  small-scale irregularity
is the opposite of smoothness.  I don't know what to make of this.  More
information is required.

For that matter, "smooth" is not necessarily the same as rigid.  So
it requires another leap to arrive at the assumption that there is
a _single_ point of contact.

We don't even know whether it is an elastic collision or not.

As I previously stated, I'm not sure the question, as stated, makes sense.
So far, all we have is a word game disconnected from physical reality.

I assume there is an underlying real physical situation and a nontrivial
reason for asking the question.  Until we find out what that is, I have
no idea how to answer the question.

I've seen plenty of physics students who think they know all about
collisions, yet have no idea how to play billiards.  Collisions between
3D objects -- even _regular_ 3D objects -- are quite a bit more complicated
than collisions between point particles.  A useful starting point is the
treatise by Ron Shepard:
  http://www.tcbilliards.com/articles/physics.html