The effects of momentary stresses in metals.

In 1872 the late Dr. John Hopkinson published an investigation into the effect of a blow delivered by a falling weight on the lower and free end of a wire, the upper end of which is fixed. It is unnecessary to repeat the mathematical analysis in full, but its main features appear in the following argument:—As soon as the weight strikes the stop at the lower end a wave of extension starts up the wire, and the velocity with which it is propagated is √E/ρ = a, where E is Young’s modulus, and ρ the density of the wire. At a time t after the weight has struck, so short that its velocity is not appreciably diminished, the lower end of the wire has moved through a distance Vt, where V is the velocity of the weight immediately after striking.