GPE & KE (Edexcel GCSE Physics)

• 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:

ΔGPE = m × g × Δh

• 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

• 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 = ½ × m × v²

• 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: