How do you calculate the kinetic energy of a moving object?
Short answer
Kinetic energy = ½ × mass × speed², written Eₖ = ½mv². Use mass in kilograms and speed in metres per second to get the energy in joules (J). Because speed is squared, doubling the speed multiplies the kinetic energy by four.
Step by step
- Convert mass to kg and speed to m/s. To change km/h to m/s, divide by 3.6.
- Square the speed first.
- Multiply by the mass, then halve the result.
- Give the answer in joules, or kilojoules for large values.
Worked example
Find the kinetic energy of a 70 kg cyclist moving at 36 km/h.
- v = 36 ÷ 3.6 = 10 m/s
- v² = 100
- Eₖ = ½ × 70 × 100 = 3500 J = 3.5 kJ
Common mistakes
- Squaring the whole product mv instead of only v.
- Using km/h instead of m/s.
- Forgetting the ½.
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