All moving objects have kinetic energy, E_{k}.

curriculum-key-fact
The KE of an object depends on its:
  • mass
  • speed

Calculating kinetic energy

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

Kinetic energy = \frac{1}{2} x mass x (speed)2

Kinetic energy = \frac{1}{2} mv2

or

E_{k} = \frac{1}{2} mv2

where:

E_{k} = kinetic energy in joules, J

m = mass in kg

v = speed in m/s

Question

What is the kinetic energy of a 1000 kg car travelling at 5 m/s?

E_{k} = \frac{1}{2} mv2

m = 1000 kg

v = 5 m/s

E_{k} = \frac{1}{2} 1000 kg x (5 m/s)2

E_{k} = 12,500 J

The car has 12,500 J of kinetic energy

Question

A car of mass 1200 kg, travelling at a steady speed, has a kinetic energy of 175 kJ. What is the speed of the car?

E_{k} = \frac{1}{2} mv2

The car has 175 kJ of kinetic energy. This must be converted into J to use in the equation for kinetic energy.

175 kJ = 175,000 J

E_{k} = 175,000 J

m = 1200 kg

175,000 = \frac{1}{2} x 1200 kg x v2

175,000 = 600 kg x v2

v2 = \frac{175,000 J}{600 kg}

v2 = 291.67

v = \sqrt{291.67}

v = 17.1 m/s

The car has a speed of 17.1 m/s.