# Friction

A force along a surface that resists sliding. Its largest value is the coefficient of friction times the normal force.

Physics (university) · Part 02: Newton's laws and forces · Card 7 of 10 · Last updated 2026-10-19

## Analogy

Sandpaper grips more than ice.

## Example

**Starting to slide**

N = mg = 196 N
f max = 0.4 x 196
f max = 78.4 N

## In practice

A 20 kg box with a friction coefficient of 0.4 needs about 78 N to start sliding.

## Practice

### Flashcards

- **What is the formula for maximum static friction?** f max = (coefficient of friction) x (normal force).

### Interview questions

**A 10 kg block slides down a 30 degree ramp with friction coefficient 0.2. Find its acceleration.**

- Draw a free-body diagram
- Weight component along slope is mg sin theta
- Friction is coefficient times mg cos theta
- Net force over mass gives acceleration

## Questions people ask

### What is the difference between static and kinetic friction?

Static friction holds an object that is not sliding. It can be any size up to a maximum. Kinetic friction acts once sliding starts and is usually a bit smaller. This is why it takes more effort to start moving a heavy sofa than to keep it moving.

### Does friction depend on contact area?

In the simple model used in first-year courses, no. Friction depends on the coefficient and the normal force. A brick on its wide face or narrow face has about the same friction. Real surfaces are more complex, but this model works well for most problems.

## Related concepts

- [Normal force](https://studyvisually.com/learn/science/physics-university/physics-university-part-2/physics-university-part-2-concept-6)
- [Free-body diagram](https://studyvisually.com/learn/science/physics-university/physics-university-part-2/physics-university-part-2-concept-8)
- [Inclined plane](https://studyvisually.com/learn/science/physics-university/physics-university-part-2/physics-university-part-2-concept-10)
- [Newton's first law](https://studyvisually.com/learn/science/physics-university/physics-university-part-2/physics-university-part-2-concept-1)

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