Kinetic Energy

Kinetic energy is the energy that an object possesses because it is moving. Any moving object has this energy; when motion stops, the energy becomes zero. Its magnitude is measured in the standard unit of energy in the SI system, which is the joule (J).

Kinetic energy depends on two things: the mass of an object and its velocity. Velocity has a squared effect: if velocity doubles, kinetic energy increases fourfold. For this reason, a fast-moving vehicle releases far more energy when brought to a stop.

Basic formula

In classical physics, for an object of mass m moving at velocity v, kinetic energy is calculated as: E = ½ m v². This formula applies when velocity is much lower than the speed of light. When velocity approaches the speed of light, a formula based on special relativity is used, which shows that energy increases without limit as velocity approaches the speed of light.

Relation to work

The work-energy theorem states that the total work done by forces on an object equals the change in its kinetic energy. When a force pushes an object and increases its velocity, positive work is done; when a force such as friction opposes motion, negative work is done and kinetic energy decreases, often converting to heat.

Types of motion

A rigid body can possess kinetic energy that is translational and kinetic energy that is rotational. Rotational kinetic energy depends on rotational velocity and the distribution of mass relative to the axis. For gas molecules, the sum of the kinetic energy of each molecule is related to temperature; thus higher temperature corresponds to faster molecular motion.

Energy conversion

Kinetic energy continuously converts to other forms. An object falling from a height converts potential energy to kinetic energy. A water turbine or wind turbine converts the kinetic energy of water or air into electrical energy. In a system with no external losses, the total energy is conserved, though its forms change.