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h1. Hooke's Law for elastic interactions

{excerpt}A mathematical approximation to the restoring behavior of springs and other elastic solids under small deformations.{excerpt}

h3. Motivation for Concept

Elastic objects are objects which rebound to their original shape from a temporary deformation.  All solids are basically elastic under small deformations, but "small" is a relative term.  A metal ball, for instance, is only elastic for deformations that are so small as to be invisible to the naked eye.  If you press the ball hard enough to deform it noticeably, it will retain a dent.  A rubber ball is elastic under a much wider range of deformations, and can have its shape noticeably changed temporarily without causing a permanent dent.    

The elastic properties of objects is vital to understanding the engineering of all structures, from airplanes to skyscrapers.  As such, Hooke's description of the restoring force produced by an object undergoing _elastic deformation_ is an extremely useful piece of mathematics, and has acquired the title "Hooke's Law", even though it is not a universal Law in the same sense as, e.g. [Newton's 2nd Law|Newton's Second Law] or his law of [universal gravitation|gravitation (universal)].  Hooke's "Law" is really a parameterization which is only valid for deformations small enough that the object is in the elastic regime.  This fact does not detract from its enormous utility, since keeping structural members in an elastic state is often a goal in engineering.  Objects experiencing elastic deformation are said to "obey Hooke's Law".  In introductory mechanics, Hooke's Law is most frequently used to describe the resoring force of springs, which are objects designed to "spread out" large deformations over a series of coils, so that the complete object can change shape dramatically while each portion of the coil deforms only a relatively small amount.

h3. Hooke's Law in terms of Force

h4. Mathematical Statement of the Law

As applied to springs, Hooke's Law is generally stated for a spring which has one end fixed.  For that case, the restoring force acting on the other end of the spring when it is moved by stretching or compressing the spring along its length (taken to be the x-direction) will be given by:

{latex}\begin{large}\[ \vec{F} = -k(x-x_{0})\hat{x}\]\end{large}{latex}

where _x_~0~ is the natural position of the end of the spring that is being moved.  


h4. Elasticity and Simple Harmonic Motion

The elastic restoring force fulfills the conditions for [simple harmonic motion].  This can be seen for the case of a spring held fixed at one end.  The defining relationship for simple harmonic motion in the _x_ direction is:

{latex}\begin{large}\[ \frac{d^{2}x}{dt^{2}} = - \omega^{2}x \]\end{large}{latex}

where ω (which is the angular frequency of the resulting motion) is not a function of _x_.  

This relationship will be present for a mass attached to a spring that is fixed on the other end.  Hooke's Law tells us:

{latex}\begin{large}\[ ma_{x} = - kx \]\end{large}{latex}

where we have assigned coordinates such that _x_~0~ = 0.  

By the definition of acceleration:

{latex}\begin{large}\[ a_{x} = \frac{d^{2}x}{dt^{2}} = - \frac{k}{m}x\]\end{large}{latex}

which is in the form of the simple harmonic motion equation with

{latex}\begin{large}\[ \omega = \sqrt{\frac{k}{m}}\]\end{large}{latex}

h3. Elastic Potential Energy

Assuming an object attached to a spring that obeys Hooke's Law with the motion confined to the _x_ direction, it is customary to choose the coordinates such that _x_ = 0 when the object is in a position such that the spring is at its natural length.  The force on the object from the spring is then:

{latex}\begin{large}\[ \vec{F} = - kx \hat{x} \]\end{large}{latex}

It is also customary to make the assignment:

{latex}\begin{large}\[ U(0) \equiv 0\]\end{large}{latex}

Thus, the potential can be defined:

{latex}\begin{large}\[ U(x) = U(0) - \int_{0}^{x} (-kx)\:dx = \frac{1}{2}kx^{2}\]\end{large}{latex}

For an object moving under the influence of a spring only, the associated potential energy curve would then be:

!epe.gif!

The graph indicates the presence of one stable equilibrium point at _x_ = 0.

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