# [ODE] Little physics problem

Eike Decker eike at cube3d.de
Fri Nov 9 09:33:09 MST 2007

```As ODE doesn't simulate air resistance, this behavior is physically correct when
the rolling ball does not loose energy in a different way. As far as I
understand it, the friction does not change the energy of the body - because
the speed on the surface of the ball at the contact point should be 0 if it is
rolling properly. In that case, friction cannot absorb energy as the energy at
the contact point is 0.

Modeling air resistance is complicated if you want to make it precise, the most
simple way is to put a damping force (roughly speed = speed*.999) on all
bodies. Depending on your wanted behavior, you might want to implement a more
complex damping formula which reflects more the shape and size of the body.

Eike

> Did you set the 'mu' field in contacts to a non zero value?
> E.g.:
> contact[i].surface.mu = 100.0;
>
>   Bram
>
>
> On Nov 9, 2007 6:48 AM, Joao Pereira <joaopapereira at gmail.com> wrote:
>
> > i have a ball and a plane.
> > i release the ball on the plane and the place is horizontal so the ball
> > don't move.
> > after some time i apply a force (0,10,0) on the ball and the ball starts
> > moving. no problem the ball starts gaining velocity has it should.
> > When i stop applying the force the velocity drop, has it should, but
> > after a little time the velocity get stable and don't get down to 0, has
> > i studied in physic's it should.
> > So where is the force that the plane "apply's" that should stop the
> > ball???????
> > How can i model the force applied by the air over my objects??? there
> > must also be a resistance there, that i don't see.
> >
> > Thanks for the answer
> >
> > João Pereira
> > _______________________________________________
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> > ODE at ode.org
> > http://ode.org/mailman/listinfo/ode
> >
>
>
>
> --
> Zapp: Captain's log, stardate...er..
> Kif: Ohhh. April 13th.
> Zapp: April 13th. Point 2.
>

```