GPE & KE (Edexcel GCSE Combined Science: Physics): Revision Note

Gravitational Potential Energy

• Energy in the gravitational store of an object is defined as:

The energy an object has due to its height in a gravitational field

• This means:

  • If an object is lifted up, energy is transferred to its gravitational potential store 

  • If an object falls, energy will be transferred away from its gravitational potential store

Energy is transferred to the gravitational potential store of the mass as it is lifted through a gravitational field

• The change in the gravitational potential energy, ΔGPE of an object can be calculated using the equation:

$∆GPE=m\times g\times ∆h$

• Where:

  • ΔGPE = change in gravitational potential energy, in joules (J)

  • m = mass, in kilograms (kg)

  • g = gravitational field strength in newtons per kilogram (N/kg)

  • Δh = change in vertical height in metres (m)

• In Physics, Δ is the capital Greek letter 'delta' which stands for 'change in'

• The two graphs below show how GPE changes with height for a ball being thrown up in the air and when falling down

Graphs showing the linear relationship between GPE and height

Kinetic Energy

• Energy in an object's kinetic store is defined as:

The amount of energy an object has as a result of its mass and speed

• This means that any object in motion has energy in its kinetic energy store

• Kinetic energy can be calculated using the equation:

$KE=\frac{1}{2}\times m\times v^2$

• Where:

  • KE = kinetic energy in joules (J)

  • m = mass of the object in kilograms (kg)

  • v = speed of the object in metres per second (m/s)

Energy Equivalency

• In a perfect energy transfer, there is no wasted energy

• Energy transfers can be assumed to be perfect if the wasted energy transfer is negligible

  • Some exam questions will state to ignore air resistance for example

  • In reality, there is no such thing as a perfect energy transfer

• Ignoring wasted energy transfers is helpful in calculations because it allows energy values to be equated

• Pendulums are often used as examples of perfect energy transfers

  • All of the energy in the kinetic store of the pendulum is transferred mechanically into its gravitational potential store

  • And then all of the energy in the gravitational potential store of the pendulum is transferred mechanically into its kinetic store

  • Energy is transferred back and forth between these two stores as the pendulum swings

  • Therefore, it can be said that:

$K{E}_{total}=GP{E}_{total}$