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What is kinetic energy?

Kinetic energy is the energy an object has because of its motion, equal to half its mass times its velocity squared.

Kinetic energy explained

The velocity is squared, so doubling speed quadruples kinetic energy — the reason braking distance grows so sharply with speed.
In energy questions, kinetic energy usually converts to or from gravitational potential energy, with any shortfall dissipated as heat by friction or air resistance.

Key formula

Ek=12mv2E_{k} = \tfrac{1}{2}mv^{2}

Worked example

A 70 kg70\,\text{kg} runner at 5 m s−15\,\text{m s}^{-1} has 875 J875\,\text{J} of kinetic energy.

Examiner tip

Square the velocity before multiplying, and never square the mass. Half of all lost marks in this topic are that single slip.

Kinetic energy: common questions

Is kinetic energy a vector?
No. Energy is a scalar, so kinetic energy has magnitude only and is never negative.
How does kinetic energy relate to braking distance?
The work done by the braking force equals the kinetic energy removed, so distance is proportional to speed squared.

Related terms

Momentum

Momentum is the product of an object's mass and velocity, and it is conserved in any collision or explosion with no external force.

Conservation of energy

The principle of conservation of energy states that energy cannot be created or destroyed, only transferred between stores.

Work done

Work done is the energy transferred when a force moves an object through a distance in the direction of the force.

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