What is kinetic energy?

Energy associated with motion

Kinetic energy is the energy possessed by an object as a result of its motion. A stationary object has zero kinetic energy; as soon as it starts to move, it gains kinetic energy. The faster an object moves, and the greater its mass, the greater its kinetic energy.

The formula

For an object of mass m (in kg) moving at a speed v (in m/s), the kinetic energy Ec is given by:

Ec = 1/2 * m * v^2

It is measured in joules (J), the unit of energy in the International System of Units.

A central role in physics

Kinetic energy is involved in everything: car crashes, the motion of planets, the flight of aeroplanes, or even the movement of a simple ball. It helps us understand why a lorry travelling at 90 km/h is far more dangerous than a bicycle travelling at the same speed: mass plays a direct role.

Example

A cyclist and their bicycle, with a total mass of 80 kg, are travelling at 5 m/s.

Ec = 1/2 * 80 * 5² = 1/2 * 80 * 25 = 1000 J

Common pitfall

Be careful not to forget the factor 1/2, and above all, do not confuse v and v²: doubling the speed does not double the kinetic energy; it is multiplied by 4!