# Physics 311, Extra Credit for Week #7: due Apr 30, 1999

Is centripetal force really equivalent to gravitational force?

This project may be done by teams of 1-2 individuals.

Your task in this assignment is to measure the maximum force which can be sustained by a thread or string before it breaks. You should do it in two ways: first, by hanging weights from the thread until it breaks: this will yield the maximum gravitational force it can endure. Second, by swinging objects around and around by the thread until it breaks: this will yield the maximum centripetal force it can endure.

Find a source of thread or string which is strong enough to hold at least a few pounds, but weak enough that it breaks under a moderate weight (say, 20 or 30 pounds). Cut six identical lengths of this material, each about three feet long.

Use three of the pieces to determine the maximum weight the thread can hold. Attach an object to one end of the string, and tie the other end to a convenient post or beam so that it hangs straight down. Add weight to the object at the bottom until the thread breaks. Write down the amount of weight required to break the string in each trial.

Use the other three pieces to determine the maximum centripetal force the string can endure. Go outside to a large, empty area, such as the athletic fields on campus. Make sure that no bystanders are close enough to be hit by flying objects. Tie an object of moderate weight -- say, one to three pounds -- to one end of the piece. Hold the other end. Measure the distance between your hand and the object. Twirl the object in a circle around you in a horizontal plane. Start twirling slowly. Measure the number of revolutions per second -- you may use your own watch, or check out a stopwatch from the stockroom. Very gradually, increase the speed of your twirling; try to keep track of the number of revolutions per second as you go. Keep increasing the angular velocity until the thread breaks. Calculate the centripetal force exerted by the thread just before it broke. Make three trials.

Make a table, showing the results of all six trials. How does the maximum gravitational force sustained by the string compare to the maximum centripetal force? Do they differ significantly? If so, can you explain why?

Submit a report which contains pictures and descriptions of your work, as well as the results. You may also include interesting items you discovered during the course of the procedure.