Friction
A short lesson with diagrams, a worked example, a problem to try, and a quick check. Then quiz yourself on 4 practice questions.
Friction acts along the surface and opposes sliding. Kinetic friction (sliding) is μkFn. Static friction (not sliding) is only as big as it needs to be to stop the slipping, up to a maximum of μsFn.
1. Static vs. kinetic
2. Which way does friction point?
Along the surface, opposite the way the object slides, or the way it would slide if there were no friction.
A crate sliding right feels friction pointing left.
Friction depends on the normal force, not directly on the weight. Anything that changes Fn, like an angled pull or an incline, changes the friction too.
Bigger μ, more friction: rubber on dry concrete (μ ≈ 0.68) grips better than on wet concrete (μ ≈ 0.58).
3. Example, then try it
A 20 kg box sits on a level floor with μs = 0.50 and μk = 0.30. You push it horizontally with 60 N. What is the friction force?
Fn = mg = 200 N, so the maximum static friction is (0.50)(200) = 100 N.
Your 60 N push is less than 100 N, so the box doesn’t move. Static friction matches your push: 60 N, not 100 N.
Same box. Now you push with 120 N. Does it move? If so, what is its acceleration?
Compare to the maximum: 120 N is more than the 100 N maximum static friction, so the box slides.
Kinetic friction: Ff = μkFn = (0.30)(200) = 60 N, opposite the push.
Net force: 120 − 60 = 60 N in the direction of the push.
Acceleration: a = Fnet ÷ m = 60 ÷ 20 = 3 m/s².
Answer: it slides with an acceleration of 3 m/s² in the direction of the push.
4. Common mistakes
A 10 kg crate sits on a level floor with μs = 0.40. You push it horizontally with 25 N, and it doesn’t move. What is the friction force on it?
The maximum static friction is (0.40)(100 N) = 40 N, but the crate isn’t slipping, so static friction only needs to balance your push: 25 N.
Ready to practice?
4 questions on Friction, each with a hint and an explanation for every wrong answer.