# Mass-energy equivalence

Mass is a form of energy, E = m c squared. A tiny mass is a huge amount of energy.

Physics (university) · Part 10: Relativity and quantum physics · Card 4 of 11 · Last updated 2026-10-19

## Analogy

A gram of matter is like a big store of fuel.

## Example

**One gram**

E = m c^2
= 0.001 x (3e8)^2
= 9e13 J

## In practice

Converting 1 g of mass releases about 9e13 J, roughly 21 kilotons of TNT.

## Practice

### Flashcards

- **What is E = mc^2?** Mass and energy are equivalent. Rest energy equals mass times c squared.
- **Why can't an object with mass reach light speed?** Its energy would grow without limit as v approaches c.

## Questions people ask

### Where does the Sun get its energy?

In the Sun's core, four hydrogen nuclei fuse into one helium nucleus. The helium is slightly lighter, by about 0.7 percent. That missing mass becomes energy through E = mc squared. The Sun turns around four million tonnes of mass into energy every second.

### Does a hot object weigh more than a cold one?

Technically yes, because energy has mass. The change is too small to measure: heating 1 kg of water by 100 K adds 419,000 J, which is about 5e-12 kg. But in nuclear reactions the mass change is large enough to see.

## Related concepts

- [Postulates of relativity](https://studyvisually.com/learn/science/physics-university/physics-university-part-10/physics-university-part-10-concept-1)
- [Time dilation](https://studyvisually.com/learn/science/physics-university/physics-university-part-10/physics-university-part-10-concept-2)
- [Length contraction](https://studyvisually.com/learn/science/physics-university/physics-university-part-10/physics-university-part-10-concept-3)

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