Tim Nikias v2.0 wrote:
> I'm stuck: my new I/O Particle-System uses adaptive iteration to calcul
ate
> the path for a particle, along with object interaction. Now, when Parti
cles
> would normally roll on a surface, they oscillate heavily in my System:
> Gravity pulls them down, object interaction puts them back on surface, 
only
> the speed perpendicular to the surface-normal is retained. Thus, a part
icle
> won't slow down when rolling along plane, but it will "hop" on the surf
ace
> hundreds of times (two of my particles reached more than 2500 iteration
s
> that way...).
> 
> To avoid this, I want a particle to switch to "Rolling" when enough
> iterations have been done, and thus the particle will just get moved al
ong
> the surface without "real" interaction. Now, if I'd just be dealing wit
h a
> plane, that'd be no problem, but since particles may also roll uphill, 
I
> want to calculate how much velocity they'd lose when doing that. I know
 the
> angle of the surface, the initial velocity and the distance the particl
e
> will travel, now what's the formulae to calculate how much velocity is 
lost?

In a particle system you usually describe the particles as point masses 
so they won't roll (rolling means rotation and a point can't rotate). 
The effect you get (hopping) is pretty normal but it will diminish if 
you add damping (just imagine a glass marble falling on a carpet floor 
instead of a massive steel block).  Damping can be modelled in different 

ways - you can either continuously diminish the kinetic energy or you 
slow down the particles during every collision.

> For simplicities sake, use a ball with 10km/h that starts rolling a 45
> degree upward slope for 10 meters. Gravity is 10m/s. What'd be the velo
city
> after 5 meters? What's the formula for that?


y 
depends on the friction between the ball and the surface (if you really 
want it rolling you also have to take the moment of inertia into account)
.

Christoph

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