Purpose:Illustrates an inertial frame of reference
Description:A pair of spring-powered cannon firing one-inch ball bearings are lined up with holes in two plexiglass plates, one in the center and one on the side opposite the cannons. The second plate has sacks on the holes to collect the projectiles if they pass through the holes. If the frame is at rest, the projected balls fail to even go through the first set of holes because they are deflected by gravity. If the frame is raised, held in place by an electromagnet and released, it falls with the acceleration of gravity and becomes a "local inertial frame of reference." The balls are automatically fired by a gravity switch when the frame begins to fall. The balls will travel along straight lines in the local inertial frame of reference and end up in the sacks before the frame stops on the shock absorber.