Hi, here is your answer!
The relative motion of two solid objects is usually proportional
to the force which presses the surface together and also the roughness of the surfaces.
It is known as frictional force.
It is normal to the surfaces, which affects the frictional
resistance, is known as normal force, denoted by N.
So the formula is;
Frictional force = µN
µ = coefficient of friction
When sliding, put µk = coefficient of kinetic friction
When at rest, put µs = coefficient of static friction