Elastic
Energy
Elastic Potential
Energy (sometimes called 'stretch or compressed' potential energy)
is found in springs, elastic materials (balls and bands). It comes from
the structure of the materials and their desire to retain a particular
shape. It can be used as a store for energy (hence the word 'potential'). The energy stored is the area under a force/extension graph line.
Elastic Behaviour
When a material is
stretched (or compressed) but returns to its original size when the load
is removed, it undergoes elastic behaviour. Elastic bands are a good example
of this although most materials exhibit elastic behaviour to some extent.
Plastic Behaviour
When a material is
stretched (or compressed) but does not return to its original size when
the load is removed, it undergoes plastic behaviour.
Atoms that make up
a material are arranged in a particular way - when you compress or stretch a material with enough force to alter this arrangement you make the object experience plastic behaviour. You permanently deform the material.
Hooke's
Law
Hooke's Law says that
the restoring force due to a stretched (or compressed) spring is proportional
to the the change in its length (extension or compression), and acts in
the opposite direction as long as the elastic limit is not exceeded..
This means that if
you hang weights onto the end of a spring and measure how much the spring
stretches for each weight you can plot a graph of your results and show
that they are proportional to each other at first and them the proportionality
stops. Plot the extension on the x-axis against the weight applied to
the spring on the y-axis. You will get a straight line for the initial
readings and a curve for the higher ones - if you carry on and add very
heavy weights you will cause the spring to break! (fracture).
Interactive
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See also the Young Modulus