Kinetic friction — Science definition

The friction force between two surfaces that are already sliding past each other; f = μₖN.

Where it is taught

In use

Kinetic friction applies while surfaces slide and is often modeled as μkN.

Friction & Common Forces

Questions using kinetic friction

Problem 1

A block is pressed to a surface with a normal force of 50 N. If the coefficient of kinetic friction is 0.40, find the kinetic friction force as it slides.
Answer and explanation

20 N

f = μₖN = 0.40 × 50 = 20 N.

Problem 2

A 15 kg box slides across level ground with μₖ = 0.25. Find the kinetic friction force. (g = 9.8 m/s²)
Answer and explanation

36.75 N

N = mg = 15 × 9.8 = 147 N. f = μₖN = 0.25 × 147 = 36.75 N.

Problem 3

Explain the difference between static and kinetic friction, and describe a situation where you can feel the switch between them.
Answer and explanation

Static friction acts between surfaces that aren't sliding yet and adjusts up to a maximum to prevent motion; kinetic friction acts between surfaces already sliding and is a bit weaker. You feel the switch when pushing a heavy couch: it resists until you push past the static maximum, then suddenly 'gives' and slides more easily under the smaller kinetic friction.

Problem 4

A 10 kg box slides down a 30° incline where the coefficient of kinetic friction is μₖ = 0.20. Find its acceleration down the slope. (g = 9.8 m/s²)
Answer and explanation

3.2 m/s²

Along the slope: a = g(sinθ − μₖcosθ) = 9.8(0.5 − 0.20 × 0.866) = 9.8(0.5 − 0.173) = 9.8 × 0.327 ≈ 3.2 m/s². (Mass cancels out.)

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