Introduction

Every solid is built from atoms (or groups of atoms) locked into a definite geometric arrangement. Nature chooses this particular arrangement because, in it, the net force on each atom from all its neighbours is zero — this is the solid's equilibrium configuration at room temperature. The atoms are not perfectly still even at this equilibrium; they keep vibrating about their fixed positions (and continue to do so at very low temperatures too), but they cannot wander away from those positions. It is precisely this locked-in arrangement, not the individual atoms' vibration, that gives a solid its characteristic definite shape and definite size — unlike a liquid or gas, which take the shape of their container.

When an external force is applied to a solid, its constituent atoms are displaced slightly from their equilibrium positions. This displacement immediately sets up internal restoring forces that oppose the displacement and try to pull the atoms back to where they started. If the deforming force is removed, these restoring forces are (within limits) strong enough to bring the body back to its original shape and size — but this recovery is possible only up to a certain limit of applied force, not for arbitrarily large forces.

A body's overall form is described by its size and its shape — two related but distinct ideas: a tennis ball and a football have the same shape (both spherical) but different sizes. Any force that changes a solid's size, shape, or both, by shifting the relative positions of its constituent molecules, is called a deforming force, and the resulting change in the body is called deformation. In general, the larger the deforming force, the larger the resulting deformation, whether that deformation shows up as a change in a wire's length, a change in an object's volume, or a change in a body's shape.

Not every material responds to a deforming force the same way. Stretch a rubber band and it elongates, but the moment you let go it snaps back to its original length. Apply a similar stretching or squeezing force to dough or clay, however, and it stays deformed — it does not spring back once the force is removed. This contrast in behaviour between rubber-like materials and clay-like materials is what the property called elasticity (or, at the other extreme, plasticity) is used to describe, and it is the subject of the rest of this chapter.