M-D2: Force, action and reaction

PIRA: Unknown

Equipment: Spring-driven toy auto, 3 500g slotted masses, good piece of window glass about 6 x 30 inches with wrapping paper stuck to the upper surface, twine, Bunsen burner, scotch tape, Ealing-Daw air table (properly leveled). The toy auto is in Set 9, Cabinet 3, Shelf 1, the air table is in end of Set 6, Cabinet 3, the glass plate is in Room 4 on a shelf above the movies, and the air blower is on the bench (set of drawers) in Room 75.

Procedure: The purpose of this demonstration is to show more dramatically than does M-D4 and M-D5 the principle of force action and reaction. If the earth were light enough, the motor of an automobile would drive the earth backward more than the car forward; and that is what this experiment is designed to emphasize. The paper attached to the top of the the glass plate serves two purposes: (1) it makes the plate readily visible from the back of the room, (2) it prevents spinning the wheels when the car is not loaded.

The plate of glass is placed on the table, and the table is leveled (may take 10 min.) The auto, with spring wound to the degree desired, is set on the track and kept from rolling by twine attached to the glass with scotch tape and to the rear of the car with tape or by the setting of one of the 500g masses on it. When it is time to start the auto — that is, when the glass plate has been steadied to the point that it is not drifting appreciably — burn the string with the Bunsen flame. (A match will be blown out by the wind from the table, but a good cigarette lighter might serve satisfactorily. However, the Bunsen flame sometimes has trouble unless adjusted for good stability).

For maximum effect use two 500-g masses in the auto and none on the glass plate. If this is done, you must be ready to stop the auto and the plate as the auto nears the end of the plate. Otherwise, things may get out of hand. For less effect put the two 500-g masses on the glass plate, one at each end and none in the car. If time allows I prefer to do both for comparison, the lesser first. In these two specific cases the total mass of the entire system is the same. I believe it worthwhile to call this to the students' attention. If only one demonstration is to be given, I often choose one 500-g mass on the plate at the end in front of the auto and two 500-g masses in the car. This arrangement not only gives a good demonstration of the "earth's" backward movement but also shows that the total momentum of the system remains zero if you allow the auto to collide with the mass in front. This mass will slide an inch or two before bringing the auto to a stop, but when stopping is complete the whole system is at rest on the table (or drifting only slightly as it probably was at take-off).

For a picture of the accessory air pump as well as the complete air table consult the photo in M-D11.

Note: If all forces and motions are kept on the longitudinal axis of symmetry of the glass plate through its center of mass, rotation of the plate will result. This rotation will usually be too small to be noticed except by those very near the front. With care it can be avoided.