 # 2.8.3 (Unity3d): What is the exact calculation used for dynamic friction force?

In SDK 2.8.3 what is the exact algorithm used to determine the the dynamic friction force acting on a rigid body sliding over the surface? It does not seem to be a simple linear relationship. For example, the standard definition for dynamic friction if F = umg where u is the coefficient of dynamic friction, m is the mass and g is the gravity constant (-9.8 m/s-2). In this case I would have expected u to be NxMaterialDesc.dynamicFriction. However I ran some tests where I slid a rigid body with a constant mass over a surface with different starting velocities and still it stopped and used the standard kinematic equation Vf^2 = Vi^2 + 2ad and took note of d (the stopping distance). a can be replaced by F/m and F can be replaced by umg. The formula for u is then -Vi^2/(2gd). This value should be constant if u is constant however it was not constant. This suggests that frictional force is not linear but depends on the velocity of the rigid body. I also measured u with different masses and the same initial velocities and the result was the same so the frictional force does not seem to depend on mass. The documentation is not specific enough about how dynamic friction force is calculated. It simply says that increasing NxMaterialDesc.dynamicFriction makes the object stop more quickly. I need to know wxactly how an object will slide so I can make accurate predictions of object behaviour in my AI decision making code.

I meet the same problem, I want calculate the distance by the parameters in Unity3D PhysX, anyone have some advices?

I have the same problem. I’m using Unity 5.5 to calculate the rest distance of an object that has an initial velocity while sliding. I’m using these formulas given below;

Vfinal² = Vinitial² + 2(F/m)d
0 = Vinitial² + 2((µmg)/m)d
0 = Vinitial² + 2µgd == > Vinitial² = -2µgd
Distance = Vinitial² / 2µg

With last equation i am calculating the rest distance but it is not accurate. If I increase the friction constant of the surface and the object to double in the formula I can predict the rest distance with nearly %95 accuracy. Am I missing something and How can I calculate the exact Friction Force?

. can’t delete this message sorry.

. can’t delete this message sorry.

When performing your experiments take into account the direction the object is moving. If memory serves, physx’s friction is not as direction independent as you might like.

The object is moving directly and I’m calculating the friction which is the opposite direction of the movement.