Energy changes in a system

• Equation of kinetic energy


  1/2 x mass (kg) x (velocity)squared (m/s)
• Equation for elastic potential energy


  1/2 x spring constant (N/m) x (extension) squared (m)
• Equation for gravity potential energy


  mass (kg) x gravitational field strength (N/kg) x height (m)
• Define the ‘specific heat capacity’ of a substance

  The amount of energy needed to raise the temperature of 1kg of a substance by 1 degree Celsius
• Units for specific heat capacity

  J/degrees Celsius/kilogram
• Definition of power


  The rate at which energy is transferred (or rate at which work is done)
• Two equations for power

  • power = energy transferred/time
  • power = work done/time
  • power= W, energy and work done= J, time in s
• Two motors lift the same mass through the same height. motor A does this in half the time of motor B. which dissipates the most power?


  Motor A. Because the energy transferred is the same but the time taken is less
• Describe the energy changes involved when a ball is thrown upwards and then returns to its starting position. ignore air resistance

  • Upwards: KE is converted to GPE
  • peak: Maximum GPE, zero KE
  • downwards: GPE is converted to KE
• Describe the energy transfers for a bungee jumper

  • as the cord tightens, KE is converted and stored as elastic potential energy (EPE)
  • at lowest point, the jumper’s initial GPE equals the EPE stored in the cord
• Explain why a bungee jumper slows down once the cord begins to stretch

  • kinetic energy decreases since it is converted to elastic potential energy
  • since KE is proportional to (velocity) squared, as KE decreases, so does velocity
• Give examples of chemical energy stores 


  • food
  • fuels (eg wood, coal, petrol)
  • Batteries
• State 4 different energy stores


  • kinetic energy
  • gravitational potential energy
  • elastic potential energy
  • chemical energy