• When the body is about to move, the limiting friction gives F = μ R. The angle of friction λ is defined by:

tan λ = μs

• The angle of friction is the angle that the total reaction of the surface makes with the normal reaction when the body is on the point of moving.

• The coefficient of friction is never negative, and in the problems of this course it is usually not greater than 1.
• The limiting friction increases with the normal reaction, and does not depend on the area of contact.