Any moving object has energy in its kinetic energy store. The more mass it has and the faster it moves, the more kinetic energy it has. The equation is: kinetic energy (J) = ½ × mass (kg) × (speed)² ((m/s)²), or KE = ½ × m × v².
Worked examples: a 2 kg trolley moves at 3 m/s. The speed squared is 3 × 3 = 9, so KE = ½ × 2 × 9 = 9 J. A 1000 kg car travels at 20 m/s. The speed squared is 400, so KE = ½ × 1000 × 400 = 200000 J.
The speed is squared but the mass is not. If the mass is multiplied by two, the kinetic energy is multiplied by two. If the speed is doubled, the kinetic energy is multiplied by four. A common mistake is to forget to square the speed, or to square the ½ × mass × speed together instead of squaring only the speed. Kinetic energy is measured in joules.