Re: [Phys-l] Interactive Physics Simulations

[John Denker <jsd@av8n.com>]

At 09:04  -0400 10/9/06, Ron Curtin wrote:

> >  Does anyone out there know how to make the two objects
> > collide and stick together? Thanks.

On 10/09/2006 11:48 AM, Hugh Haskell wrote:

> Making perfectly inelastic collisions has always been a problem for 
> IP. If have talked to them about it and they always tell me that it 
> is a difficult programming problem and they continue to work on it.

That's funny.

This is a physics problem, not a software problem.

> From a purely software point of view, programming a macroscopic
description of an ineleastic collision is trivial.  Consider the
case of two hard spheres colliding:  Just conserve momentum
(including angular momentum) and ignore the energy equation,
i.e. thermalize however much energy you need to, in order to
make the momentum come out right.  That's all there is to it.

The only way this gets to be hard is if you try to give a
microscopic accounting for the physics, in particular for the
thermalization step.

Moving to physics now, it is AFAIK impossible to have an
instantaneous inelastic collision.  It's like trying to make
thin black paint:  basically impossible.  This is also related
to trying to "instantaneously" make the coupling between two
rail cars.  Real-world rail cars don't even try to do it
instantaneously;  they have "draft gears", as we discussed
back in November 1999 and on various occasions since.

So ..... you should plan on spreading the inelastic interaction
over some nonzero time and space.  In this case I suggest making
an L-shaped (or upside-down T-shaped) pendulum and arranging for
the foot to stab into a nonzero-sized chunk of putty.  It will
slow down bit by bit as it stabs deeper and deeper into the
putty.