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View offerAffiliate link — we may earn a commission at no extra cost to you.Calculate kinetic energy using clearly labelled inputs.
Kinetic energy is the energy an object has because it's moving:
KE (J) = ½ × mass (kg) × velocity² (m/s)²
Because velocity is squared, doubling the speed quadruples the energy.
A 1,500 kg car at 60 km/h 1. Convert speed: 60 ÷ 3.6 = 16.67 m/s 2. KE = 0.5 × 1,500 × 16.67² = 208,333 J (208 kJ)
The same car at 100 km/h 1. 100 ÷ 3.6 = 27.78 m/s 2. KE = 0.5 × 1,500 × 27.78² = 578,704 J (579 kJ), which is 2.8 times the energy at 60 km/h
A 156 g cricket ball at 40 m/s (144 km/h) 1. KE = 0.5 × 0.156 × 40² = 124.8 J
Braking has to remove all of a car's kinetic energy. Energy rises with the square of speed, so braking distance does too: going from 50 to 60 km/h increases the energy by 44%. That's the physics behind lower speed limits in busy streets.
It becomes four times larger, because energy depends on velocity squared.
Divide by 3.6. For example, 72 km/h = 20 m/s.
For a 1,500 kg car, about 579 kJ.