> Energy can't be perpendicular to anything, because it isn't a vector.
> What you mean is velocity.
>
> The reason why a ball loses energy is because of friction inside it, and
> the amount of friction is dependent on the deformation, wich is dependent
> on the acceleration/change of velocity.
>
> Probably this friction mainly (maybe only) slows down the velocity
> perpendicular to the surface, but if the ball has a velocity-component
> parallel to the surface it will slide (or roll) on it while in contact
> with it, resulting in friction between the surface and the ball wich
> slows it down in this direction, too.
>
> How about letting the user specify the ratio between horizontal and
> vertical friction?
>

Yes, this is all true, I was just trying to simplify the matter without
getting too technical.  By energy i really meant (and should have said)
momentum.  The amount of energy lost is dependent on the plasticity of both
objects involved in the collision, for simplistic purposes you can express
this as a percentage.  I also like to assign energy loss in two parts,
elastic and plastic, (maybe not the most correct terms) with elastic energy
loss being a percent and plastic being an absolute value (this keeps the
ball from bouncing forever as it would if you used a pure percentage). If
you want to use friction, I would apply it separate (not a ratio) as it
would depend on the material of the two impacting objects, and it would
apply to the parallel vectors.

Also, I have noticed I misstepped a bit in my initial explanation.  When
resolving the momentum vectors into x y z components, these are according to
a local frame of reference aligned to the plane of impact, not the global x
y z, so I suppose I should call them u v w instead, with u being the
perpedicular vector and v and w being (perpendicular) vectors lying on the
plane of impact.

Let me know if further clarification is needed.  I will explain a bit more
later when I rehash my code.

-tgq

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