Rune wrote:
> 
> So far in my particle system I have used a simple collision model where
> the outcoming angle is equal to the incoming angle and the outcoming
> velocity is smaller than the incoming velocity due to the energy loss at
> the collision. ( See my illustration I in povray.binaries.images in the
> message "Collision with energy loss" ) But I wonder if that is a very
> good model to use...
> 
> [...]

As you probably know my own physics simulations use an elasticity based
method to calculate collisions.  Therefore math is probably quite
different although there are similarities of course.  

As Scott said, the collision damping is strongly influenced by the
material properties of the colliding objects, but not necessarily only by
the elasticity.  A well known example for very few energy loss is a glass
ball on a massive metal surface, but a rubber ball (quite high elasticity)
looses much less enery than a wood ball for example.

The source of collision damping is the dissipation of energy during
deformation of objects.  Therefore you are right that shallow collisions
result in less energy loss, but you will have to consider friction and
rolling (although rolling is something not considered in a particle
system by definition since the particles have no size and therefore no
moment of inertia).

I'm not totally sure about the direction of the damping force, the method
i use in my system is having a viscous damper (i.e. force proportional to
the speed) parallel to the collision elasticity which is orthogonal to the
surface.  This is a common method, but i have to say i'm not always
content with the results of my system.  In general i would say daming
forces should always be against the direction of current speed which would
result in both an orthogonal and tangential damping component.

Christoph

-- 
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