Energy in a system can be changed, in particular by mechanical work. It cannot be created or destroyed but it can be transferred, dissipated or stored in different ways.

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

This is when:

- kinetic energy (
*KE*) is measured in joules (J) - mass (
*m*) is measured in kilograms (kg) - speed (
*v*) is measured in metres per second (m/s)

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

The amount of gravitational potential energy stored by an object at height can be calculated using the equation:

change in gravitational potential energy = mass × graviational field strength × change in vertical height

This is when:

- change in gravitational potential energy (
*∆GPE*) is measured in joules (J) - mass (
*m*) is measured in kilograms (kg) - gravitational field strength (
*m*) is measured in newtons per kilogram (N/kg) - change in vertical height (∆ℎ) is measured in metres (m)

Galileo takes a 5 kg cannonball to the top of the Tower of Pisa for one of his experiments. The tower is 56 m high. How much gravitational potential energy has the cannonball gained? (g = 10 N/kg)

- Question
How much gravitational potential energy does a 500 g book gain when it is lifted up 1.5 m onto a shelf?

For any of these equations you may need to change the subject of the formula.