Work and energy

• Work


  A measure of the energy transferred, always measured in Joules (J)
• Work


  Force × distance moved
• Spring constant


  Determines how much force is required to stretch a spring
• Calculating spring constant


  force = spring constant × extension
• When a force acts on a body


  Work is done
• In most cases the transfer of energy is not efficient, some energy is lost due to the work done by friction
• The total energy in the system must stay the same; if there is less energy at the end than at the start, some must have been lost as heat
• Car efficiency improvements


  • Start stop systems to reduce waste whilst stationary
  • Lighter materials to reduce the mass of the car
  • More streamlined shapes to reduce the work done by frictional forces on the car
  • Ensuring correctly inflated tyres to reduce the work done by frictional forces
• Calculating kinetic energy


  kinetic energy = 1/2 mass × velocity^2
• Calculating gravitational potential energy

  gravitational potential energy = mass × gravitational field strength × change in height
• Calculating energy stored in a spring

  Energy stored in a spring = the area under the force extension graph
• When work is done, the total energy in the system must remain the same. The total energy at the start must equal the total energy at the end of the action.
• Energy transfer examples


  • A skier with 12,000J of gravitational potential energy at the top of a slope (A) will have 12,000J of kinetic energy at the bottom of the slope (B)
  • A stretched slingshot stores 2J of elastic potential energy, when released the force of the rubber changing shape does work on the ball and transfers 2J of kinetic energy to the ball
• Car safety features


  • Air bags
  • Crumple zones
• These safety features change shape during a collision, this allows the work to reduce the kinetic energy of the passengers to be done over a longer distance
• The longer the distance the work is done over, the smaller the force on the passengers will be