14·1. A rigid body means a body in which the distance between each pair of particles remains invariable. The bodies with which we are familiar are all more or less elastic and capable of compression, extension or distortion under the action of external forces, and the problem of the motion of such a body is in general rendered more complicated by its deformability. The problem of the motion of bodies is greatly simplified by the hypothesis that they are rigid in the sense denned above, and in elementary dynamics we limit our considerations for the most part to the problem of the motion of one or more such bodies.

Further, we regard a rigid body as an agglomeration of particles held together by cohesive forces such that the action and reaction between any pair of particles are equal and opposite

(9.7), and we use the principles established in Chapter IX for the motion of a system of particles. In the case of the rigid body however there is an additional fact to be observed, namely, that since the distance between each pair of particles is unaltered in any displacement of the body, therefore the total work done by the action and reaction between the particles is zero.