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'Elastic' When a steel spring is extended the force extension graph is a straight line. As the spring contracts the graph returns to zero on the same straight line. Steel is said to be elastic. No energy is converted to heat in the process. |
Rubber is inelastic. The force-extension graph is not a straight line and the graph does not return to zero on the same path as the rubber contracts. Rubber is said to be inelastic. Energy is converted to heat as the rubber extends and contracts.
Area on a force-distance graph has the units of Newton meters, ie. Joules. Heat work and energy are all measured in Joules.
The work done on the rubber as it extends is the area under the upper curve.
The work done by the rubber as it contracts is the smaller area under the lower curve.
The difference is the area of the loop which is the heat generated in the rubber in Joules.
Graphs can be plotted by forcing the mass down and allowing it to return slowly by hand. The motion detector is on the floor below the shield. The areas of the hysteresis loops are almost identical, independent of the period of the cycle (at least for the rubber bands used to make these graphs.)
It would be interesting to investigate the truth of this statement for other types of rubber.
When a mass oscillates on a length of rubber the amplitude reduces. Kinetic energy is converted to heat during each cycle. The oscillation is said to be damped.
As the oscillations take place the hysteresis loops become smaller each cycle.
It is interesting to compare the kinetic energy lost per cycle with the area of each successive hysteresis loop.
> Energy loss due to hysteresis