[ODE] Little physics problem

Remi Ricard remi.ricard at simlog.com
Fri Nov 9 08:59:04 MST 2007

Hi Joao,
> Not quite sure i did understand what u said.
> i have setted the mu to 50.0 and to infinite and to 0 and the only 
> difference is the time it takes to get to the final velocity, that 
> changes too.
See the contact Joint on the wiki page.


> The ball should never be with a constant velocity when no force 
> applied and only the friction is present......

The ball with have constants velocity if there is no force applied on 
it. (This is one of the Newton's law).

Since friction is a force then if there is friction and velocity at the 
contact point the velocity can decrease.
But ...

If the ball is only rolling and not sliding the velocity at the contact 
point is zero. (This is the instantaneous point of rotation (I took my 
physics courses in French so this might not be the good translation)). 
Since the friction force is only against the velocity there is no 
friction force in that case.

Since there is also no deformation no energy is lost in Heat. (ODE is 
rigid body simulation)

So the velocity should remain constant. (Try the demo_buggy. It is not a 
sphere but 3 cylinders but in a straight line it is the same effect).

> João Pereira
> Remi Ricard escreveu:
>> Hi Joao,
>>> 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.
>> You are right
>>> When i stop applying the force the velocity drop, 
>> Yes and no.
>> - No  the velocity stay constant if there is no friction. (air, 
>> ground or whatever).
>> - Yes if you have set the mu for the contact joint and have set the 
>> good parameter also.
>>> 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.
>> See above.
>>> So where is the force that the plane "apply's" that should stop the 
>>> ball???????
>> In the contact joint ?
>>> 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.
>> You can add a force on the body with is proportional to the square of 
>> the velocity time a factor.
>> Remi

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