Question

How is the force of friction related to the normal force?

Answer 1

Through the coefficient of friction - `F_f = mu.R` where R is the reaction (normal) force and `F_f` is the frictional force.

Explanation:

Basic explanation here: http://www.school-for-champions.com/science/friction_equation.htm#.Wighc7p2t2Y

Much more detailed understanding available here: https://en.wikipedia.org/wiki/Friction