uncertainty about the force of static friction

Question

Hello,

I was wondering whether someone might be able to help me better understand static friction. Why doesn't static friction always equal μF_n? When does it make sense to assume that f = μF_n? When friction doesn't equal μF_n, what does the size of the frictional force depend on?

Any help would be much appreciated!

Answer 1

Static friction (The surface friction when an object is not sliding against a surface) is passive and responds the forces acting on the body that are trying to move the body. It's like a wall pushing back on you when you push into it. It will only exert the force back that will stop the wall from moving.

The static friction has a maximum value of μ_staticF_Normal but it will only have a value that will maintain static equilibrium (F_net=0). If I drag on a 100kg crate with μ_s = .5 and I pull with 400 Newtons it will not move. It has the capability of a F_f = .5(100kg)(10N/kg) = 500N, but it will only exert 400 Newtons back to counter the force that I am exerting (anything else would be silly, as that would mean that the crate would accelerate due to the friction force)

You actually only use the static friction equation for the limits of the forces necessary to move something. Once the object moves, you must use F_f = μ_kF_N because there is sliding taking place. Because static friction is always greater than kinetic friction you can't avoid an abrupt jump when you get something starting to move with friction present. Once the object moves you have to lower the force to go a constant velocity in order to just balance the kinetic friction force.

I hope that helps.