Kinetic energy

Kinetic energy is one type of energy store. It is the energy stored in a moving body. The amount of kinetic energy stored in a body depends on its mass and its speed.

Calculating energy in the kinetic store

The amount of kinetic energy of a moving object can be calculated using the equation:

Kinetic energy = ½ × mass × velocity2

\text E_{k} = \frac{1}{2} \text m v^{2}

This is when:

  • energy ( \text E_{k}) is measured in joules (J)
  • mass ( \text{m}) is measured in kilograms (kg)
  • velocity ( \text{v}) is measured in metres per second (m/s)

Example

An apple of mass 100 g falls from a tree. It reaches a speed of 6 m/s before landing. What is the gain in kinetic energy of the apple?

\text E_{k} = \frac{1}{2} \text m v^{2}

\text E_{k} = \frac{1}{2} \times 0.1 \times 6^{2}

\text E_{k} = \frac{1}{2} \times 0.1 \times 36

\text E_{k} = 1.8 \text J

Question

How much kinetic energy does a 30 kg dog have when it runs at 4 m/s?

\text E_{k} = \frac{1}{2} \text m v^{2}

\text E_{k} = \frac{1}{2} \times 30 \times 4^{2}

\text E_{k} = \frac{1}{2} \times 30 \times 16

\text E_{k} = 240 \text J