Kinetic energy

Definition

Kinetic energy is the energy possessed by an object as a result of its motion. The faster and more massive an object is, the greater its kinetic energy. It is denoted by Ec and expressed in joules (J).

Formula

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

Ec = 1/2 * m * v^2

This formula shows that kinetic energy is proportional to the square of the speed: if the speed doubles, the kinetic energy is multiplied by 4, not by 2.

Example

A car with a mass of 1000 kg travelling at 20 m/s (or 72 km/h) has a kinetic energy of: Ec = 1/2 * 1000 * 20² = 200,000 J = 200 kJ

Speed (m/s)	Ec for m = 1000 kg (J)
10	50,000
20	200,000
40	800,000

Pitfalls to avoid

• Forgetting the factor 1/2 in the formula.
• Forgetting to square the speed.
• Using a speed in km/h instead of m/s: you must always convert (divide by 3.6) before calculating.
• Confusing mass and weight: m is in kg, not in newtons.