Physics 11 · Physics · Lesson 3.1
Friction
- Level
- Grade 11
- Reading time
- 12 min reading
- Review
- Last reviewed 2026-09-03
- Standard
- Accuracy standard
- Errata
Abstract
Static and kinetic resistive forces.
What is friction in physics?
Short answer
Friction is a force that opposes the relative motion or attempted motion between two surfaces in contact. Static friction prevents an object from starting to move and can vary up to a maximum; kinetic friction acts on a sliding object and is roughly constant. Its size depends on the normal force and the roughness of the surfaces, expressed by a coefficient of friction.
Concept preview
What you will be able to do
- 01Describe static and kinetic friction
- 02Calculate friction with F_fr = μF_N
- 03Explain the role of the normal force and the coefficient of friction
Try to shove a heavy sofa across the floor and something pushes back — that's friction, the force that opposes sliding between two surfaces in contact. It always acts along the surface, pointing opposite to the motion (or the attempted motion). Friction is the reason a rolling ball eventually stops, the reason a pushed box doesn't slide forever, and — as we'll see — the reason you can walk at all. Look at the forces on a pushed box:
School Plan
Friction
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Related topics
Frequently asked questions
- What is the difference between static and kinetic friction?
- Static friction resists the start of motion and varies up to a maximum; kinetic friction acts once sliding begins and is roughly constant.
- What does friction depend on?
- The normal force pressing the surfaces together and the coefficient of friction set by their materials and roughness.