AQA A Level Physics

Revision Notes

4.7.6 Energy Conservation

Conservation of Energy in Deformation

Loading and Unloading a Metal Wire

  • When a metal wire is loaded with a force and stretched beyond its limit of proportionality, it will undergo plastic deformation
  • When the force is removed, the wire is unloaded, this causes the extension to decrease
  • The unloading line is parallel to the loading line (since k does not change) however, it does not go through the origin
    • If the wire is permanently deformed, it will not be at zero extension when there is no force as it is now permanently extended
  • The area between the loading and unloading lines represents the work done to permanently deform the wire

Loading and unloading graph 1, downloadable AS & A Level Physics revision notes

Force-extension graph of a material that has undergone plastic deformation

Loading and Unloading a Rubber Band

  • The force-extension graph for a material may not always be the same when loading (adding a force) and unloading (removing a force)
  • The force-extension curve for stretching and contracting a rubber band is shown below

Loading and unloading graph 2, downloadable AS & A Level Physics revision notes

Loading and unloading on a force-extension graph

  • Since the rubber band has no extension when the load is fully removed, it has no permanent extension
    • This means that the rubber band is elastic
  • The graph shows the rubber band stores a greater amount of strain energy when it is loaded (stretched) than when it is being unloaded (contracted)
  • The curve for contraction is always below the curve for stretching
  • The key features of the area under the graph are:
    • Area is the work done in heating the rubber (or the increase in thermal energy)
    • Area Y is the work done by the rubber when it is returned to its original shape
    • Area X + Y represents the work done in stretching the rubber band originally
  • However, due to the conservation of energy, the difference in strain energy when loading and unloading must be accounted for
  • A rubber band becomes warm when it is stretched and contracted hence some energy is transferred to heat energy

Energy Conservation Issues

  • Vehicle suspension systems are made up of tires, springs and shock absorbers which provide comfortable handling of a vehicle and improve the comfort of passengers
  • Roads are often very bumpy filled with potholes and speed bumps
    • A bump in the road causes the wheel of a vehicle to move up and down perpendicular to the road surface.
    • If a wheel loses contact with the road surface, it will slam back down again causing large vibrations within the car and potentially damage the vehicle
    • As well as this it would be very uncomfortable for the passengers and the driver could lose control of the vehicle

Shock-Absorber, downloadable AS & A Level Physics revision notes

Vehicle suspension, springs and shock absorbers above the wheel help absorb any impact forces

  • This energy is absorbed by shock absorbers
    • These are elastic objects designed to absorb or dampen the compression and rebound of the springs above a vehicle’s tires
    • They help keep the tires on the road at all times
  • When a vehicle hits a bump in a road, the shock absorbers dampen the movement of the springs in the suspension system
    • They do this by converting kinetic energy, from the movement of the car, into thermal energy which is dissipated
  • The faster the springs in the suspension system move (say, if a vehicle hits a bump at a high velocity), the more resistance the shock absorber provides

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Ashika graduated with a first-class Physics degree from Manchester University and, having worked as a software engineer, focused on Physics education, creating engaging content to help students across all levels. Now an experienced GCSE and A Level Physics and Maths tutor, Ashika helps to grow and improve our Physics resources.
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