# Kinetic energy

The energy of motion, KE = 1/2 m v squared. Doubling speed quadruples it.

Physics (university) · Part 03: Work, energy and momentum · Card 2 of 9 · Last updated 2026-10-19

## Analogy

A moving bowling ball can knock pins because it carries energy.

## Example

**Car at 20 m/s**

KE = 1/2 m v^2
KE = 0.5 x 1000 x 400
KE = 200,000 J

## In practice

A 1000 kg car at 20 m/s carries 200,000 J.

## Practice

### Flashcards

- **What is the formula for kinetic energy?** KE = 1/2 m v^2.

### Interview questions

**A 1500 kg car travelling at 15 m/s hits a stationary 1000 kg car and they lock together. Find their speed and the energy lost.**

- Use momentum conservation
- Combined mass is 2500 kg
- Compare kinetic energy before and after

## Questions people ask

### Why is kinetic energy proportional to speed squared?

Kinetic energy equals the work needed to reach a speed. With a steady force, twice the speed takes twice the time, and the distance grows with time squared, so it is four times longer. Work is force times distance, so energy is four times bigger. So braking distance at 60 km/h is about four times that at 30.

### Can kinetic energy be negative?

No. Mass is positive and speed squared is never negative, so KE is zero or positive. Only changes in energy can be negative. If an object loses kinetic energy, its change is negative, but its actual kinetic energy always remains zero or above.

## Related concepts

- [Momentum](https://studyvisually.com/learn/science/physics-university/physics-university-part-3/physics-university-part-3-concept-7)
- [Collisions](https://studyvisually.com/learn/science/physics-university/physics-university-part-3/physics-university-part-3-concept-9)
- [Work](https://studyvisually.com/learn/science/physics-university/physics-university-part-3/physics-university-part-3-concept-1)
- [Work-energy theorem](https://studyvisually.com/learn/science/physics-university/physics-university-part-3/physics-university-part-3-concept-3)

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