Identify the two surfaces in contact
Determine the normal force
Find the coefficient of friction
Use the friction formula: friction force = coefficient of friction × normal force
Distinguish between static friction and kinetic friction
For static friction, compare the applied force to the maximum static friction
For kinetic friction, use the kinetic friction coefficient
Measure or calculate the force needed to start or maintain motion
Check the direction of the friction force, which opposes relative motion or attempted motion
Verify the result using units of force, such as newtons