Identify the type of friction: static, kinetic, or rolling
Use the formula for kinetic friction: (F_f = mu_k N)
Use the formula for maximum static friction: (F_f le mu_s N)
Use the formula for rolling friction if given: (F_f = mu_r N)
Find the coefficient of friction (mu) from the problem or reference data
Find the normal force (N)
For a horizontal surface, use (N = mg) if no other vertical forces act
For an inclined surface, use (N = mg costheta) if no other forces act perpendicular to the surface
Multiply the coefficient of friction by the normal force
For static friction, compare the applied force to (mu_s N)
If the applied force is less than or equal to (mu_s N), friction equals the applied force
If the applied force is greater than (mu_s N), use kinetic friction (mu_k N)
Keep units in newtons (N)