PURPOSE: Illustrates what is meant by a "local inertial frame of reference."

DESCRIPTION: Cannons 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 holes have sacks to collect the balls 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. Click cannon #1 to see the front cannon fired, cannon #2 to see the rear cannon fired, or both cannons to see both cannons fired simultaneously. Short videos from the point of view of an observer in the reference frame are also available for cannon #1 and cannon #2.

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. See the path of each ball taken using a camera mounted in the local inertial frame of reference by clicking your mouse on either of the two images below. (Notice the apparent motion of the meter stick behind the local inertial frame as the frame falls.) Click on the large image above to see a slow-motion movie of the entire frame in free-fall. Click here to see the camera falling off of the falling reference frame.

SUGGESTIONS:

See Question of the Week #224 for information on using this demonstration to enhance class involvement.

REFERENCES: (PIRA unknown.) See Demonstration Reference File for article describing this apparatus as well as articles on other aspects of local inertial frames of reference.

EQUIPMENT: Local inertial frame of reference with two balls.

SETUP TIME: None.