Revision Note 5: Friction

Frictional force (more correctly, the maximum frictional force) is normal force times coefficient of static or dynamic friction (as the case may be).

F_fr = μN

The normal force (N) that is used to compute frictional force is best computed by considering the force by the surface on the body – in other words you need the free-body diagram of the body. This is the best method to compute the force by the body on the surface, as by Newton’s third law the two forces are numerically the same.

Problem Solving Tips:

Tip 5.1:

What is F_fr  for a ramp of incline θ to horizontal when the coefficient of friction is μ? By a previous tip (Tip 2.3), N = mg.cosθ. So,

F_fr,θ = μN = μ mg.cosθ

Tip 5.2: Up-down intuition:

Relating θ with normal force, N, and frictional force, F_fr,θ:

θ ↑        N ↓      F_fr,θ  ↓

The effect of friction decreases as the ramp becomes more and more vertical.