friction

i have a question. How does friction work from static to dynamic.

I mean if an object is standing still. and a force F is applied to it and it still stand still cus of friction R. This means R=-F
So ok. But when will it start to move? I heard people say when F is higher than Rmax. But what is Rmax? is it Rmax=uN? where u is the static friction coefficient?
And when it moves, friction will always be R=uN?

Yes. Rmax = uN, where Rmax is maximum static friction, u is the coefficient of static friction, and N is the normal force.

When an object is moving use it's coefficient of kinetic friction; when it's stationary, use it's coefficient of static friction until it starts moving.

that's correct :)
R = ud*N where ud = Dynamic friction coefficient. As an additional note, dynamic friction coefficient is less or equal than static friction.

Think of what happens when you try to push a heavy desk or a box. You push, and push, but it doesn't move. Then you push a bit harder and it suddenly gives and you fall flat on your face. That's because non-moving objects have a higher coefficient of friction than moving ones. All you need to do is say "If abs(speed) <= (small value) then use static friction coefficient else use dynamic friction coefficient"

Modeling friction is just a matter of:
a) calculating your forces, including friction
b) canceling forces with friction
c) applying the remaining forces to speed
d) canceling speed with any remaining friction

hm the coefficient is just a number, so not big problem. i was just unsure of the rules of static friction. Since static friction R is not uN but is equal to F until it come to it's max value Rmax