# Centripetal force

The net inward force needed for circular motion, F = m v squared / r. It is a role played by tension, friction or gravity.

Physics (university) · Part 04: Rotation and gravity · Card 3 of 11 · Last updated 2026-10-19

## Analogy

Not a new force, just a job given to an existing one.

## Example

**Friction on bend**

F = m a
F = 1000 x 4.5
F = 4500 N

## In practice

A 1000 kg car on that bend needs 4500 N of sideways friction.

## Practice

### Flashcards

- **What gives a satellite its centripetal force?** Gravity from the planet.

### Interview questions

**Derive the speed of a satellite in circular orbit and use it to find the period of a satellite at radius r.**

- Gravity equals m v^2 / r
- v = sqrt(GM/r)
- Period = 2 pi r / v

## Questions people ask

### Is centripetal force a new type of force?

No. It is the name for the net inward force, whatever supplies it. For a car on a flat bend it is friction. For a ball on a string it is tension. For a satellite it is gravity. Never add it as an extra force on a diagram.

### What happens if the centripetal force suddenly disappears?

The object flies off in a straight line along the tangent to the circle, not outward along the radius. If you let go of a swinging ball on a string, it moves along the direction it was travelling at that moment, as Newton's first law says.

## Related concepts

- [Orbital speed](https://studyvisually.com/learn/science/physics-university/physics-university-part-4/physics-university-part-4-concept-9)
- [Universal gravitation](https://studyvisually.com/learn/science/physics-university/physics-university-part-4/physics-university-part-4-concept-8)
- [Angular velocity](https://studyvisually.com/learn/science/physics-university/physics-university-part-4/physics-university-part-4-concept-1)
- [Centripetal acceleration](https://studyvisually.com/learn/science/physics-university/physics-university-part-4/physics-university-part-4-concept-2)

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