# 3D Coulomb Friction - Use Collision Impulse To Calculate Friction Force?

I've been using this document to learn how to implement a very basic physics engine. The section on Coulomb Friction on page 55-56 (marked as pages 43-44) confuses me.

It observes that the friction force can be calculated using the collision impulse (jr). This seems strange. Envision an object falling very quickly onto a plane with a slight incline. The object is rotating in the opposite direction it would if it were rolling down the incline. When the object makes contact, it will have a small velocity along the tangent, but a huge normal impulse (jr). This will result a very large change in velocity along the tangent (t hat) opposite to the direction of motion. This will push the object up along the surface (bad!) with a notable amount force. My implementation confirms this.

Can someone explain to me how this all fits together? I'm very confused.

P.S. I don't think trying to provide an example of my implementation here is relevant, either the effect I'm explaining cannot occur (in which case there is a bug, which I can find myself), or I'm misunderstanding the equations, in which case the implementation is inherently broken

• It looks like in the case you described the force would fall in the static friction cone, and be balanced. Also, as an aside, take care to distinguish between angular momentum and linear momentum. – Jay Jan 6 '18 at 9:26
• @Jay when it comes down to the friction, its the velocity of the actual contact point, which is entirely linear, the angular momentum gets converted when you are no longer at the center of mass. Also, the static friction is only in effect when there is no relative velocity, but in this case, there is a huge amount of relative velocity. – GiantCowFilms Jan 6 '18 at 18:01
• I interpreted the static friction as occurring when the relative velocity falls in the static cone. In your question the relative velocity is small but in your comment it's huge. Should very fast spinning object be expected to create a lot of friction? – Jay Jan 7 '18 at 2:50
• @Jay A spinning object can be subjected to a lot of friction force due to its spinning, it takes a good deal of strength to hold an angle grinder in place when you push hard. Also, I was unclear in saying there was a huge amount or relatively velocity (most of that is on the up down axis), my point is that vr is not zero because of that, and some of it translates into the tangent vector, due to the spinning. The case is designed to have as little velocity along the tangent as possible, while having the largest normal force. – GiantCowFilms Jan 7 '18 at 3:46
• If you look at equation 2.69, its visible that the dynamic friction is only visible if vr (relative velocity) is not 0. It does not take into account how large vr is. – GiantCowFilms Jan 7 '18 at 3:49