Hello,
In btRigidBodyConstructionInfo, we can specify a m_friction for objects.
The only help in the doc is here:
http://bulletphysics.org/Bullet/BulletF ... c7ad43a0bf
"best simulation results when friction is nonzero "
But what is exactly the unit of this?
Thank you
Friction: what is the unit?
 drleviathan
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 Location: San Francisco
Re: Friction: what is the unit?
Friction has no unit of measure; it is a dimensionless coefficient. Just think of it as a dial that twist from 0 (frictionless) to 1 (max friction).
Note that the effective friction of a collision between two objects is actually the average of the two objects friction coefficients.
Note that the effective friction of a collision between two objects is actually the average of the two objects friction coefficients.
Re: Friction: what is the unit?
OK, but what does this number mean?
Re: Friction: what is the unit?
theoretically speaking max parallel force <= mu (that number) * perpendicular force. If you press down with 0.5N of force and the combined mu is 0.5, the maximum friction force will be 1N. Bullet friction is a bit off the theoretical mark due to approximations, but more or less that.Gregwar wrote:OK, but what does this number mean?
Last edited by Basroil on Tue Jan 05, 2016 2:11 am, edited 1 time in total.
Re: Friction: what is the unit?
You mean the maximum friction force will be 0.5N?Basroil wrote:theoretically speaking max parallel force <= mu (that number) * perpendicular force. If you press down with 1N of force and the combined mu is 0.5, the maximum friction force will be 1N. Bullet friction is a bit off the theoretical mark due to approximations, but more or less that.Gregwar wrote:OK, but what does this number mean?

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Re: Friction: what is the unit?
It is indeed dimensionless, but there's no upper limit.drleviathan wrote:Friction has no unit of measure; it is a dimensionless coefficient. Just think of it as a dial that twist from 0 (frictionless) to 1 (max friction).
The friction value that you set on a rigid body object in Bullet isn't a "real" physical property (and isn't what's normally called the coefficient of friction)  it's just subsequently used to derive a friction coefficient for the resulting contact. The friction coefficient for the contact, when multiplied by the normal contact force, gives the maximum frictional force at that contact, as Basroil wrote.
That's not correct (except in special cases). The default calculation of the friction coefficient in Bullet looks like this:Note that the effective friction of a collision between two objects is actually the average of the two objects friction coefficients.
Code: Select all
btScalar btManifoldResult::calculateCombinedFriction(const btCollisionObject* body0,const btCollisionObject* body1)
{
btScalar friction = body0>getFriction() * body1>getFriction();
All this applies to the restitution value too.