calculating forces during collisions

[Varun V Prabhu]

I want to output to the console window, the force experienced when an
object collides with the other. I tried the "dReal * dBodyGetForce
(dBodyID);" command in the boxstack demo provided with ODE but it
outputs only zeroes even when a selected object collides with the
other. How can I find this out in general for any body ?

[Daniel K. O.]

It works like this:

* forces are accumulated by the user through dBodyAddForce. Those are
the "external forces".

* you call dWorldQuickStep or dWorldStep.

* gravity is also accumulated as an external force

* the solver computes the joint forces so the body movements satisfy the
constraints. Each joint with a dJointFeedback set will store this
constraint force on the feedback.

* the accumulated forces are zeroed.

* the step function returns.

So you can only know the force applied by the contact joints through
joint feedbacks. That, or use the change of velocity of the bodies
multiplied by their mass to estimate the impulse, and divide it by the
timestep size to estimate the force.

--
Daniel K. O.

[Varun V Prabhu]

the dGetLinearVel() function returns zero when an object is under the
influence of gravity i.e falling freely. How can I know the velocity
during this time ?

[Jon Watte]

dGetLinearVel() function returns zero when an object is under the
influence of gravity

That should not be true.

Sincerely,

jw

--
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[Daniel K. O.]

Em 15-06-2010 10:59, Jon Watte escreveu:
>
> dGetLinearVel() function returns zero when an object is under the
> influence of gravity
>
>
> That should not be true.

Right now I'm under the influence of gravity, and my linear velocity is
zero. ;-)
The question is, if the object is free-falling, did you step the world
at all?

--
Daniel K. O.