That's actually a pretty difficult question. I
guess the best way to answer that is to say that
any time a material is deformed or forced out of
it's natural, at-rest state the deformation falls
into 1 of two categories: elastic or plastic. An
elastic deformation is defined as one where the
material will return to it's original shape after
the force has been removed while a plastic
deformation causes permanent change in the
material. Imagine squeezing a rubber
bouncy-ball...after you squeeze it, the ball
returns to it's original shape. This is an
elastic deformation. If you were to try the same
thing with a ball of clay it would not return to
it's original shape. This is plastic deformation.
This can also be seen by bending a pen or a
pencil. A slight bend and it will recover it's
shape but if you go too far it will either break
or stay bent. This is another important point.
Most materials have both an elastic and a platic
range of deformation, meaning initially the
deformation is elastic but if you push them too
far the deformation will become
Springs are specially designed to
permit large amounts of elastic deformation. The
first element of this design is the coiled shape
that you see in most springs. This shape allows
the spring as a whole to either be compressed or
stretched very far while each section of the
spring only undergoes a small change in shape. In
this way, large deflections are possible but the
material stays within the elastic region of
deformation. Springs are also made of special
materials, usually spring steel, which are
designed to undergo large elastic deformation.
The last thing I might point out is that any
spring will deform plastically if pushed too far.
This is easily seen in the small springs in some
ball-point pens if you pull on them rather than
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