Topic 8 - Forces doing Work

• Work done


  Equal to energy transferred whenever energy is transferred, work is done
• Work done has the same unit as energy transfer which is the Joule
• Equation for work done

  Work done = Force x Distance (in the direction parallel to the force)
• Power


  The rate of energy transfer, measured in Watts (J/s)
• Efficiency = Useful energy out / Total energy in x 100
• Efficiency can also be calculated using power: Useful power out / Total power in
• Energy cannot be created or destroyed, only transferred from one form to another
• Energy stores


  • Gravitational potential energy
  • Kinetic energy
  • Elastic potential energy
  • Thermal energy
  • Chemical energy
  • Nuclear energy
  • Electrostatic energy
  • Magnetic energy
• Energy transfers: light, sound, heat, electricity
• Gravitational potential energy (GPE)

  GPE = mass x gravitational field strength x change in height
• Kinetic energy (KE)


  KE = 1/2 x mass x velocity^2
• GPE lost


  KE gained (in a perfect system)
• Efficiency is always less than 100% due to energy dissipation, usually as heat
• Ways to improve efficiency: reduce friction (lubrication), use low resistance wires