National Aeronautics and Space Administration

Gravity, whether it is holding you to the Earth’s surface or swallowing light in a black hole, is the same force obeying the same laws at all places in the observable Universe. We can measure it with this simple lab.

PURPOSE: To confirm the numerical value of g using two different experimental methods.

MATERIALS: Part A: tuning forks with frequencies between 256 - 384 Hz; adding machine tape; carbon paper; masking tape; hooked 100 g or 200 g masses; ruler or meter stick. Part B: ring stand or other device to support a pendulum; one meter of string per lab station; a pendulum bob, or hooked mass; a stop watch.

PROCEDURE - Part A

Measure a length of paper tape slightly greater than the distance from the lab bench to the floor. Make several folds in the end of the paper tape, and secure it with masking tape. Poke a hole through the reinforced end of the paper tape and insert the hook of the laboratory mass. Tape a piece of masking tape or other smoother tape to the edge of the lab bench. Cut a piece of carbon paper which is the same size as the width of the paper tape with the height of the edge of the lab bench. Stick this piece of carbon paper onto the sticky side of another piece of tape so the part of the carbon paper that makes it black is facing the edge of the lab bench. Loosely tape the carbon paper over the other piece of tape. Thread your paper tape between the carbon paper and the taped edge of the lab bench. Hold the paper tape so that the mass is hanging freely just below the edge of the lab bench. Strike the tuning fork on a rubber mallet or the heel of your hand. Touch the edge of the vibrating tuning fork to the tape that has the carbon paper on it and release the paper tape so that the hooked mass falls to the floor. The tuning fork must stay in contact with the carbon paper during the entire fall. The vibrating tuning fork should make a series of dots on the paper tape. These dots should be increasingly farther from each other. It may take several tries to acquire a suitable tape. For a good analysis, the tape needs to have at least 50 dots in a straight line. Results are better with 60 dots, because the first few dots cannot be used, nor can dots that are equidistant be used.

After a suitable tape has been obtained, students should label with an A one of the first dots that is clearly formed near the beginning of the tape. Count 10 spaces between the dots and mark the end of the tenth space with a B. Continue marking every tenth dot with a letter until the end of the tape is reached or the dots become equidistant. With a ruler or meter stick measure the distances AB, AC, AD, AE, AF, AG, etc. Record these measurements in a data table with the frequency of the tuning fork you used.