Energy calculations

Three formulas dominate this topic. Memorise them with units and rearrange confidently.

Kinetic energy

KE = ½ × m × v²

• KE = kinetic energy in joules (J)
• m = mass in kilograms (kg)
• v = speed in metres per second (m/s)

Doubling the speed quadruples the KE (because v is squared) — a key conceptual point in exam questions about braking distance.

Gravitational potential energy

GPE = m × g × h

• GPE = gravitational potential energy (J)
• m = mass (kg)
• g = gravitational field strength = 10 N/kg on Earth (Edexcel uses 10, not 9.81)
• h = vertical height gained (m)

GPE is relative: it depends on the chosen zero (e.g. ground level).

Specific heat capacity

Q = m × c × ΔT

• Q = energy transferred (J)
• m = mass (kg)
• c = specific heat capacity (J/kg·°C)
• ΔT = temperature change (°C)

Specific heat capacity is the energy needed to raise 1 kg of a substance by 1 °C. Water has a high SHC (4200 J/kg·°C), which is why it is used as a coolant.

Energy conservation between forms

Use conservation of energy when an object swaps energy between stores (no friction stated):

• A 2 kg ball dropped from 5 m: GPE → KE.

  • GPE at top = 2 × 10 × 5 = 100 J.
  • KE at bottom = 100 J → ½ × 2 × v² = 100 → v² = 100 → v = 10 m/s.

• A heater raises 0.5 kg of water from 20 °C to 80 °C.

  • Q = 0.5 × 4200 × 60 = 126 000 J = 126 kJ.

Edexcel exam tip

Marks are usually split into method (substitution) + manipulation + final answer (with unit). Always:

1. Write the formula.
2. Show the substitution with numbers.
3. Give the final answer with the correct unit.

Drop the unit and you drop the A1 mark even if the number is right.