The friction force is proportional to the normal force and depends on the coefficient of friction between the surfaces.
Symbols
T
friction force
[N]
μ
coefficient of friction
N
normal force
[N]
Where it comes from
Friction depends only on the normal force and the type of surfaces (the coefficient μ), not on the contact area. It acts opposite to the motion (or to the tendency to move).
When to use it
Motion along a surface; on a horizontal surface N=mg, on an inclined plane N=mgcosα.
Example
A block presses on the ground with a force of 50 N, with a coefficient of friction μ=0.3. Find the friction force.