How it works
Kinetic energy is the energy of motion: half the mass times velocity squared. The squared term is the punchline — double your speed and you carry four times the energy, which must all go somewhere when you stop.
That v² is the single most important number in road safety. A crash at 100 km/h releases 4× the energy of one at 50 km/h, and 2.8× the energy of one at 60 km/h.
Kinetic energy converts freely into other forms: heat in your brake discs, deformation in a crash, height when a ball rolls uphill.
Worked example
Calculate the joules
A 1,500 kg car at 25 m/s (90 km/h) carries KE = ½ × 1,500 × 25² = 468,750 J, about 469 kJ. Halve the speed to 12.5 m/s and the energy drops to a quarter — roughly 117 kJ — because the velocity is squared. A 0.01 kg bullet at 400 m/s carries ½ × 0.01 × 400² = 800 J, comparable to the car at just 1 m/s despite weighing 150,000× less.
Use it in real life
Driving: braking distance scales with v² — at double the speed you need roughly four times the distance to stop.
Cycling and e-scooters: a 100 kg rider+bike at 25 km/h carries ~2,400 J; at 45 km/h it's ~7,800 J.
Energy storage: flywheels store grid energy as pure kinetic energy — tonnes of steel spinning at tens of thousands of RPM.
Frequently asked questions
Why is kinetic energy proportional to velocity squared?
Because energy is force × distance, and at higher speed an object covers more distance while decelerating under the same force. Integrating F = ma over distance yields exactly ½mv².
How much kinetic energy does a car have?
A 1,500 kg car at 90 km/h (25 m/s) carries 468,750 J — the energy of roughly 112 food-calories, or a 100 g hammer travelling at 3 km/s.
Where does kinetic energy go when I brake?
Almost entirely into heat in the brake discs and pads. Electric cars recapture a large share of it back into the battery via regenerative braking — physics-driven savings.