IMPACT, the action which results on the coming together of two bodies, one or both of them in motion. If an ivory ball fall on a mar ble table which has a thin coating of oil, after impact the ball is found to have on its surface a patch of oil, which shows by its size that the ball must have been compressed at the moment of impact. When two bodies come into colli sion they compress each other at the points that touch until they have each the same velocity; during this time of compression each body acts upon the other with exactly the same impulse, the momentum lost by the one being gained by the other; if now the bodies are perfectly in elastic no further mutual rebound will take place; if the bodies are elastic they will regain their old shape, and the mutual impulsive forces of restitution will cause separation. The im pulsive forces of restitution are found to be less than those of compression; that is (see IM PULSE), the momentum lost or gained by either of the bodies during the second or restitution part of the impact is less than the momentum lost or gained in the first or compression part i of the impact in a certain ratio which is called the elasticity of the bodies. In a perfectly
elastic body this ratio would he equal to 1, in a perfectly inelastic body it is 0.
Thus, when one ivory ball comes into direct collision with another of equal size at rest, the first comes to rest and the second moves in the direction of motion of the first before impact, but with a slightly diminished velocity. When two equal ivory balls come together with equal and opposite velocities each returns on its old path with a velocity slightly lessened. When one perfectly inelastic body overtakes or meets another directly the common velocity after im pact is equal to the sum or difference of momenta of the bodies before impact divided by the sum of the masses.
In a collision of two balls not perfectly elas tic it may be shown that the total energy of motion (see ENERGY) of the two balls after collision is less than it was before, some of it having been converted into heat.