���˿���

It is useful to know how much energy is transferred from one store to another. Different equations are used, depending on how energy is transferred or stored.

An object raised above ground level

An object raised above ground level moves through the Earth's gravity field.

In a gravity field:

potential energy (J) = mass (kg) × height (m) × gravitational field strength (N/kg)

Example

A cricketer throws a ball of mass 0.16 kg upwards. It falls back down to the ground from a height of 4.0 m. Calculate the amount of energy transferred. (Gravitational field strength, g = 10 N/kg.)

potential energy = mass × height × gravitational field strength

= 0.16 × 4.0 × 10

= 6.4 J

A moving object

For a moving object:

kinetic energy (J) = 0.5 × mass (kg) × (speed)² (m/s)

Example

A racecar has a mass of 500 kg. Calculate its kinetic energy when it travels at 60 m/s.

kinetic energy = 0.5 × mass × (speed)²

= 0.5 × 500 × 60²

= 250 × 3,600

= 900,000 J

This is 900 kJ or 0.9 MJ.

A stretched spring

For a stretched spring:

energy transferred in stretching (J) = 0.5 × spring constant (N/m) × (extension)² (m)

Example

A spring is stretched elastically by 10 cm. Calculate the energy transferred if its spring constant is 200 N/m.

10 cm = 10/100 = 0.10 m

energy transferred in stretching = 0.5 × spring constant × (extension)²

= 0.5 × 200 × 0.10²

= 100 × 0.01

= 1.0 J