Impact, in dynamics, signifies a sudden interchange of the amount of motion of two bodies which meet. One of the bodies may be at rest, as when a hammer strikes a chisel. In this case the momentum of the hammer-head seems to be entirely lost, but actually it is transmitted through the chisel to the bodies with which the chisel is in contact. Or the two colliding bodies may both be in motion and out of contact with others. In this case the fact that the total momentum is unchanged by impact may be much more readily proved experimentally. It must be remembered that the momentum of a. body is measured by the product of its mass and its velocity, and that the momentum becomes of opposite sign algebraically when the direction of motion is reversed. Whether bodies are elastic or inelastic, i.e. whether they clingtogether after impact and so move as one, or whether they are separated by the internal forces of restitution that tend to make the distorted bodies recover their original shape - the same law holds good that the momentum of the whole system is unaltered by such an action. This maybe inferred from Newton's Third Law of Motion. [Dynamics.] But the degree of elasticity possessed by the colliding system will determine their relative speeds after impact, when the bodies are not inelastic, and will settle the distribution of momentum. The relative speed after impact is always less than before; kinetic energy is, in fact, lost as such and converted into heat, for the bodies are always warmer after such an interchange of momenta. The fraction of the original value, by which the relative speed is reduced, is called the coefficient of restitution, and is a measure of the mutual elasticities of the substances.