Kinetic energy

All moving objects have kinetic energy (KE). The KE of an object depends on its:

  • mass
  • speed

Elastic potential energy

A force acting on an object may cause the shape of an object to change. Elastic objects can store elastic potential energy if they are stretched. For example, this happens when a catapult is used.

Changing shape

Elastic objects can also store elastic potential energy when they are squashed. For example, this happens when a squash ball is dropped onto a hard surface.

Work is done on an elastic object when its shape changes and it stores elastic potential energy. It is possible to calculate the amount of energy stored when stretching or squashing an elastic material, but you do not need to be able to do this for your examination.

Calculating kinetic energy

Here is the equation for calculating kinetic energy.

\text{KE}=\frac{1}{2}{\text{mv}^{2}}

or

\text{KE}=\frac{1}{2}\times{\text{m}}\times{\text{v}^{2}}

where

KE is the kinetic energy in joules (J)

m is the mass in kilograms (kg)

v is the speed in metres per second (m/s)

Question

What is the kinetic energy of a 1,000 kg car travelling at 5 m/s?

KE = ½ × 1,000 × 25 = 500 × 25 = 12,500 J