I am working on a physics simulator that I am also trying to make into a game. I am having a lot of trouble with friction between spheres and a surface they are resting on which can rotate and translate. I am not sure if my model for the friction is wrong, or the code which implements it is wrong, but the spheres seem to slide back onto the rotating platform when the should be sliding off. I am using a fourth order Runge-Kutta integrator to solve the equations of motion. I would really appreciate it if anyone has any suggestions for this. Here is model for the friction: If sphere is on surface and not acceleration off of it and not moving relative to it { Static friction provides tangential and centripetal acceleration so that the sphere does not move relative to the surface. If the force required to do this exceeds the maximum force of static friction, the force is removed and the sphere begins to slide } Otherwise if the velocity of the point of contact is not zero { A sliding friction force is applied at the point of contact and is in the opposite direction of the relative velocity of the point of contact. The relative velocity is = linear velocity of sphere + angular velocity x vector to point of contact – velocity of the surface at the point under it } Otherwise if the sphere is rolling { Apply rolling friction at the point at the top of the sphere, to slow it down without causing it to slide again } Thanks in advance.

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Original post by bcnyz
Otherwise if the velocity of the point of contact is not zero

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This should be the relative velocity.

Perfect spheres are always in a situation of static friction because the point of contact does not move relatively to the surface (it does not slide).

I don''t really see where the problem is, but if I could suggest something: try isolating the problem. Apply one rule at a time and see if it does as predicted. It''s hard to help you since the problem might be in the implementation of your ideas.

Good luck,

Cédric

I am assuming that they are not perfect spheres and that there is more than one point of contact. That way, when the platform is rotating, it will cause the spheres to rotate about the axis perpendicular to the platform, which will make it look more realistic. Differences in the relative velocity at several points close to the point of contact will cause the sphere to eventually have the same angular velocity perpendicular to the platform as the platform has on the same axis.

Does anyone know of any good resources for friction with spheres? And yes I have tried google.

You should apply classical newtonian mechanics.

The friction force is

f = N * u, where N is the normal force and u is the friction coefficient. Google for static friction and kinetic friction (I''m not sure that kinetic is the right word in English).

With this, you should be able to find the torque caused by the friction force.

Cédric