Hooke's Law
A mathematical approximation to the restoring behavior of springs and other elastic solids under small deformations.
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. 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 often said to "obey Hooke's Law". 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.
Hooke's Law in terms of Force
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:
\begin
[ \vec
= -k(x-x_
)\hat
]\end
where x0 is the natural position of the end of the spring that is being moved.
Elasticity and Simple Harmonic Motion
Elastic deformations will fulfill the conditions for Simple Harmonic Motion when the force producing the deformation is removed. 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:
\begin
[ \frac{d^
x}{dt^{2}} = - \omega^
x ]\end
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:
\begin
[ ma_
= - kx ]\end
where we have assigned coordinates such that x0 = 0.
By the definition of acceleration:
\begin
[ a_
= \frac{d^
x}{dt^{2}} = - \frac
x]\end
which is in the form of the simple harmonic motion equation with
\begin
[ \omega = \sqrt{\frac
{m}}]\end
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:
\begin
[ \vec
= - kx \hat
]\end
It is also customary to make the assignment:
\begin
[ U(0) \equiv 0]\end
Thus, the potential can be defined:
\begin
[ U
= U(0) - \int_
^
(-kx)\:dx = \frac
kx^
]\end
For an object moving under the influence of a spring only, the associated potential energy curve would then be:
The graph indicates the presence of one stable equilibrium point at x = 0.