Identify the type of friction: static, kinetic, or rolling
Determine the normal force, N
Find the coefficient of friction, μ
Use the appropriate formula:
Static friction: f_s ≤ μ_s N
Maximum static friction: f_s,max = μ_s N
Kinetic friction: f_k = μ_k N
Rolling friction: f_r = μ_r N
If the object is on a horizontal surface, use N = mg
If the object is on an inclined plane, use N = mg cosθ
Substitute the values into the correct formula
Solve for the friction force, f