In testing wood the compression of the substance of the steel striking-weight may be neglected, since the steel is very hard in comparison with the wood.

In addition to the compression of the fibres at the surface of contact resistance is also offered by the inertia of the particles there, the combined effect of which is a stress at the surface of contact often entirely out of proportion to the compression which would result from the action of a static force of the same magnitude.

It frequently exceeds the crushing strength at the extreme surface of contact, as in the case of the swaging action of a hammer on the head of an iron spike, or of a locomotive wheel on the steel rail.

This is also the case when a bullet is shot through a board or a pane of glass without breaking it as a whole.
(_b_) It may move the struck body as a whole with an accelerated velocity, the resistance consisting of the inertia of the body.
This effect is seen when a croquet ball is struck with a mallet.
(_c_) It may deform a fixed body against its external supports and resistances.

In making impact tests in the laboratory the test specimen is in reality in the nature of a cushion between two impacting bodies, namely, the striking weight and the base of the machine.