Kinetic Energy
The energy of something moving. Solve for the energy, the mass or the speed — and see how much a fall would have to be to give the same energy.
What is Kinetic Energy?
Kinetic energy is the energy of motion. It grows with the square of the speed: a car at 100 km/h carries four times the energy it has at 50 km/h, which is why its braking distance is four times as long.
It is the energy you have to supply to get something moving, and the energy that has to go somewhere — into heat in the brakes, or into a crumple zone — when it stops.
Live Graph
Eₖ ∝ v²Everyday Kinetic Energies
rounded| Object | Mass and speed | Ek |
|---|---|---|
| Tennis serve | 57 g at 200 km/h | 88 J |
| Jogger | 70 kg at 10 km/h | 270 J |
| Rifle bullet | 8 g at 900 m/s | 3.2 kJ |
| Car in town | 1500 kg at 50 km/h | 145 kJ |
| Car on the motorway | 1500 kg at 120 km/h | 833 kJ |
Worked Techniques
The brakes must remove all the kinetic energy. With the same braking force, the distance grows with the square of the speed.
Rearranged, the formula tells you how fast a given amount of energy can make something move.
Two objects with the same momentum do not have the same energy: the lighter one carries more. That is why a light bullet is more dangerous than a heavy ball with the same momentum.
Sources & Verification
- D. Halliday, R. Resnick & J. Walker, Fundamentals of Physics, 10th ed., Wiley, 2014 — Ch. 7 (Kinetic Energy and Work).