Physicists keep track of energy by saying which store it is in. The ones you need:

• kinetic: anything moving
• gravitational potential: anything raised up
• elastic potential: anything stretched or squashed
• thermal: anything warm (the hotter, the more)
• chemical: fuels, food and batteries
• also magnetic, electrostatic and nuclear

Energy moves between stores in four ways: mechanically (a force moves something), electrically (a current), by heating (hot to cold) and by radiation (light and infrared). Sound waves carry energy away too.

A system is just the object or group of objects you choose to look at. In a closed system, nothing gets in or out, and the total energy never changes. Energy cannot be created or destroyed: it is only moved from store to store.

"The energy is used up" is never right. When a car brakes, its kinetic energy goes into the thermal stores of the brakes and the air. It is still there, just spread out and no longer useful.

The faster something moves and the heavier it is, the more energy is in its kinetic store.

kinetic energy = 0.5 × mass × (speed)²
E_k = ½ m v²

Energy in joules (J), mass in kilograms (kg), speed in metres per second (m/s). AQA, Edexcel and OCR all expect you to recall this one: it is not on the equation sheet.

1. A 1200 kg car moves at 15 m/s.
2. E = ½ × 1200 × 15²
3. E = 0.5 × 1200 × 225
4. E = 135 000 J (135 kJ)

Only the speed is squared. Double the speed and the kinetic energy goes up four times. Double the mass and it only doubles.

Masses in grams must become kilograms first: 57 g = 0.057 kg.

To find a speed, rearrange before you put numbers in.

1. A 60 kg runner has 2430 J of kinetic energy. How fast is she going?
2. ½ m v² = E, so v² = 2E ÷ m
3. v² = 2 × 2430 ÷ 60 = 81
4. v = √81 = 9 m/s

The most common slip is stopping at v² = 81. If your "speed" is huge, check you took the square root.