Experiment of the Month

Rotational Equilibrium

Dr. Cooney's Machine

Dr. Cooney modified our Atwood's machine pulley to create an interesting rotational equilibrium experiment. The local name for the new apparatus is "Cooney's machine."

The new addition is the wooden rod, fastened to the pulley, extending along a pulley radius. Changing the adjustable mass brings the system to a new equilibrium, with a new value for the distance, L, between the strings.

We have presented the machine to the students as a puzzle, telling them to measure L for a variety of masses, over as wide a range as possible. With R as the radius of the pulley, we have them plot adjustable mass versus L, and ask them to find the reason for the graph to be a straight line.

As part of their analysis, they predict that the slope of the graph will be m/R, where m is the value of the constant mass. The intercept is predicted to be -m. With careful handling, it is possible to see the effect of the mass of the wood (about 9 grams, written on the wooden rod in ink) on the slope of the graph.