AP Physics 1 · Unit 2, Part 1 · Forces

Friction Practice Problems

Choose the correct friction model before calculating a force. Practice breakaway limits, skidding, rough ramps, angled pushes and pulls, and friction in a connected system.

12 questions6 conceptual · 6 calculation

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Simulation scene from the Friction lesson. — A scene from the related lesson. Open the practice to work through the questions.

What you’ll practice

Decide what friction must do in the situation

Distinguish static and kinetic friction

Find the actual static friction below breakaway and compare it with the maximum available value. Use kinetic friction when surfaces slide.

Find the normal force first

Account for upward pulls, downward pushes, and ramp contact before calculating a friction limit or sliding force.

Connect friction with motion

Calculate a skidding vehicle’s stopping behavior, compare rest and sliding in a connected system, and analyze the angle at which a ramp loses static support.

A question from this set

Static friction does not always reach its limit

A box stays at rest while a 12.0 N horizontal push acts to the right. Its maximum available static friction is greater than 12.0 N. What is the actual friction force?

It equals the maximum static friction because the box is at rest.
It is 12.0 N leftward, just enough to balance the push.
It is zero because nothing is sliding.

Try it in the practice set

A 12.0 N push acts on a box that remains at rest.

Key ideas

A limit and an actual force are different quantities

Static friction adjusts up to a maximum

For surfaces that remain at rest relative to one another, the required friction may be below μₛN. Solve the force balance and check it against the available maximum.

Kinetic friction uses the sliding contact

In this introductory model, sliding friction has magnitude μₖN and opposes relative sliding at the contact. The normal force can change when another force has a vertical component.

See the physics

Compare below-limit and sliding trials

Check your answer, then vary between the prepared pushes or contact conditions. Compare a box that remains still with a sliding object, and inspect the friction and normal-force values after running.

Use a hint for a starting point or open a worked solution to follow the reasoning. Your progress saves in this browser, so you can return to unfinished questions.

Ready for friction?

Begin with a conceptual question, then work through the calculations.

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