Kinetic & Gravitational Potential Energy (AQA AS Physics): Revision Note

Kinetic & gravitational potential energy

Kinetic energy

• Kinetic energy is the energy an object has due to its motion (or velocity)

  • The faster an object is moving, the greater its kinetic energy

• When an object is falling, it is gaining kinetic energy since it is gaining speed

  • This energy is transferred from the gravitational potential energy it is losing

• Kinetic energy can be calculated using the following equation:

$E_k=\frac{1}{2}m{v}^2$

• Where:

  • $E_k$ = kinetic energy (J)

  • $m$ = mass (kg)

  • $v$ = velocity (m s^–1)

Gravitational potential energy

• Gravitational potential energy is energy stored in a mass due to its position in a gravitational field

  • If a mass is lifted up, it will gain gravitational potential energy

  • If a mass falls, it will lose gravitational potential energy

• The equation for gravitational potential energy for energy changes in a uniform gravitational field is:

$∆E_p=mg∆h$

• Where:

  • $∆E_p$ = change in gravitational potential energy (J)

  • $m$ = mass (kg)

  • $g$ = gravitational field strength (9.81 N kg^–1)

  • $∆h$ = change in height (m)

• The potential energy on the Earth’s surface at ground level is usually taken to be equal to zero

  • However, any position can be taken as zero when calculating the change in gravitational potential energy

• This equation is only relevant for energy changes in a uniform gravitational field (such as near the Earth’s surface)

Gravitational potential energy v height graphs

• The two graphs below show how the gravitational potential energy changes with height for a ball being thrown up in the air and then falling down (ignoring air resistance)

• Since the graphs are straight lines, gravitational potential energy and height are said to have a linear relationship

• These graphs would be identical for gravitational potential energy against time instead of height