For the equation μ=Fk/Fn, where Fk is kinetic force, Fn is normal force, and μ is the coefficient of friction

Question

How do you find Fk? Is this the same way to find Fs in the equation μ=Fs/Fn for static friction?

How do I find Fk if I'm only given Fn and not μ (so I can't rearrange it)?

Answer 1

If you are not given the coefficient of friction, you can still find the force of kinetic friction assuming that you know a few things. You need to know the actual acceleration of the object, and you need to know every force acting on the object with the exception of the frictional force, which is what you are interested in. You also need to know the mass of the object. The mass of the object times its acceleration gives the net force acting on it. From this net force you need to subtract all the other forces. what is left should be the frictional force, which is what you are interested in.

Answer 2

Fk = Fn * μ.
Fk is the force that friction opposes to the movement. You apply a force F, for instance, then you have a resulting force

Fr = F - Fk = F - μ*Fn.

This Fr is the one to apply the Newton second law:

Fr = m * a (m is mass, a is acceleration)

These equations are valid when there is movement. If not, then you have the static friction force.

Answer 3

If μ=Fk/Fn, then you can rearrange the formula so Fk = Fn x μ