Physics - Limitations of Hooke's Law
Limitations of Hooke's Law
Hooke's law  applies only up to a point called the _proportional
limit._ For changes in length above the proportional limit, the force is no
longer directly proportional to changes in length. There is another
important limit called the elastic limit. For changes in length up to the
elastic limit,  if the deforming force is removed, the object will
return to its original dimensions. It behaves _elastically._ For changes in
length greater than this, the object will be permanently changed.
It may 'spring back' to some extent, but not to its original dimensions.
This is shown in the diagram by the dotted line. Note that its gradient is
also _k._ The point where it meets the _x-axis_ shows the extent to which
the object's length has been permanently changed. This permanent change is
called plastic deformation. 
For example, a car that crumples in a collision has undergone a plastic
deformation since it does not 'spring back' to its original length.
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