# Conservation of energy

Energy is never created or destroyed. It changes form, and the total stays the same in a closed system.

Physics (university) · Part 03: Work, energy and momentum · Card 5 of 9 · Last updated 2026-10-19

## Analogy

Moving money between pockets does not change your total.

## Example

**Dropped ball**

m g h = 1/2 m v^2
v = sqrt(2 x 9.8 x 10)
v = 14 m/s

## In practice

A ball dropped from 10 m hits the ground at about 14 m/s, ignoring air.

## Questions people ask

### Where does energy go when friction acts?

It turns into thermal energy, which warms the surfaces and the air a little. The total energy is still conserved, but the mechanical part (kinetic plus potential) shrinks. This is why rubbing your hands warms them and why a rolling ball slows down.

### Is a perpetual motion machine possible?

No. Real machines always lose some energy as heat, so they cannot keep running forever without an energy source. A machine that made extra energy would break the conservation law, which has passed every experiment so far. Such claims always hide a flaw or a hidden battery.

## Related concepts

- [Work](https://studyvisually.com/learn/science/physics-university/physics-university-part-3/physics-university-part-3-concept-1)
- [Kinetic energy](https://studyvisually.com/learn/science/physics-university/physics-university-part-3/physics-university-part-3-concept-2)
- [Work-energy theorem](https://studyvisually.com/learn/science/physics-university/physics-university-part-3/physics-university-part-3-concept-3)
- [Gravitational potential energy](https://studyvisually.com/learn/science/physics-university/physics-university-part-3/physics-university-part-3-concept-4)

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