Question

What is frictional force and how is it expressed in a formula?

Answer 1

Friction is a force that is created whenever two surfaces move or try to move across each other. Friction always opposes the motion or attempted motion of one surface across another surface. Friction is dependant on the texture of both surfaces.

friction = μNfriction

 μNμ = coefficient of friction
μk = coefficient of kinetic friction
μs = coefficient of static friction

Answer 2

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