# Kinetic energy

The energy of a moving object. Kinetic energy = 0.5 x mass x speed squared.

Physics (school) · Part 03: Energy, work and power · Card 3 of 8 · Last updated 2026-10-19

## Analogy

The harder a ball is thrown, the more it can knock over.

## Example

**Cyclist**

m = 80 kg, v = 5 m/s
KE = 0.5 x 80 x 5 x 5
KE = 0.5 x 80 x 25
KE = 1,000 J

## In practice

A lorry at the same speed has far more kinetic energy than a bicycle, which is why it takes longer to stop.

## Practice

### Flashcards

- **Give the kinetic energy formula.** KE = 0.5 x mass x speed squared.

### Interview questions

**Explain why a bouncing ball does not return to its original height.**

- The ball has gravitational energy at the top
- Some energy becomes thermal and sound on impact
- Total energy is conserved but not all stays useful
- So the next bounce has less kinetic energy

## Questions people ask

### Why does doubling speed quadruple kinetic energy?

Speed is squared in the formula KE = 0.5 x m x v squared. Doubling v makes v squared four times as big, so the energy is four times as big. This is why a crash at 60 km/h is far more damaging than one at 30 km/h.

### What are the units in the kinetic energy formula?

Use mass in kilograms and speed in metres per second. The answer comes out in joules. If a question gives a mass in grams or a speed in km/h, convert them first, or the final energy will be wrong by a large factor.

## Related concepts

- [Conservation of energy](https://studyvisually.com/learn/science/physics-school/physics-school-part-3/physics-school-part-3-concept-2)
- [Energy stores and transfers](https://studyvisually.com/learn/science/physics-school/physics-school-part-3/physics-school-part-3-concept-1)
- [Gravitational potential energy](https://studyvisually.com/learn/science/physics-school/physics-school-part-3/physics-school-part-3-concept-4)
- [Work done](https://studyvisually.com/learn/science/physics-school/physics-school-part-3/physics-school-part-3-concept-5)

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