Forces and Elacstcity

• To change the shape of an object, more than one force must be applied, e.g. a spring must be pulled from both ends to stretch it.

  If the object returns to its original shape after the forces are removed, it was elastically deformed.

  If the object does not return to its original shape, it has been inelastically deformed.
• The extension of an elastic object is directly proportional to the applied force, i.e. they have a linear relationship and produce a straight line on a force-extension graph.

  However, once the limit of proportionality has been exceeded:

  • doubling the force will no longer exactly double the extension
  • the relationship becomes non-linear
  • a force-extension graph will stop being a straight line.
• • The spring constant indicates how easy it is to stretch or compress a spring - the higher the spring constant, the stiffer the spring.
• • A force that stretches or compresses a spring stores elastic potential energy in the spring.
  • The amount of work done and the energy stored are equal, provided the spring does not go past the limit of proportionality.
• RP: sample method

  1. Set up the equipment as shown.
  2. Add 100g (1N) to the mass holder.
  3. Measure the extension of the spring and record the result.
  4. Repeat steps 2 to 3 for a range of masses from 1N to 10N.
• RP: considerations and errors

  The extension is the total increase in length from the original unloaded length. It is not the total length or the increase each time.

  • Adding too many masses can stretch the spring too far, which means repeat measurements cannot be made.
• RP Variables:

  • The independent variable is the one deliberately changed - in this case, the force on the spring.
  • The dependent variable is the one that is measured - the extension.
• RP Hazards:

  The biggest hazard in this experiment is masses falling onto the experimenter's feet. To minimise this risk, keep masses to the minimum needed for a good range of results.
• RP: set up