Any moving object has energy in its kinetic energy store. The faster it moves and the greater its mass, the more kinetic energy it has. The equation is: kinetic energy = ½ × mass × (speed)2, or KE = ½ × m × v2.
Kinetic energy is measured in joules (J), mass in kilograms (kg) and speed in metres per second (m/s). Remember to square the speed first, then multiply by the mass, then halve it.
Mass and speed do not affect kinetic energy in the same way. Doubling the mass doubles the kinetic energy. But because speed is squared, doubling the speed makes the kinetic energy four times bigger. This is why fast vehicles are so much harder to stop.
Example: a 2 kg ball moves at 3 m/s. Speed squared = 9, so KE = ½ × 2 × 9 = 9 J. A 4 kg trolley moving at 5 m/s has speed squared = 25 (m/s)2, so KE = ½ × 4 × 25 = 50 J. A 1000 kg car at 20 m/s has KE = ½ × 1000 × 400 = 200 000 J.