M-EngMom9: Potential energy and neutral equilibrium

PIRA: Unknown

Equipment: wooden parallelogram arrangement, two similar iron balls with eyes that fit over the dowel rods of the equipment. The location of the equipment is in Set 10, Cabinet 1, Shelf 1.

Procedure: Describe and demonstrate fully the linkages of the equipment. Then ask the students what will happen if the similar balls are placed one on the horizontal rod as near the parallelogram as possible, the other on the opposite horizontal rod as far from the parallelogram as possible. Most will expect the far out ball to "overbalance" the inner ball and hence descend. Nothing happens. Demonstrate by moving the parallelogram into different configurations that true neutral equilibrium exists. Next, move the far out ball in, and the in ball far out, on their respective rods. Still neutral equilibrium. To show the balls are really alike, exchange them and repeat both parts of the experiment again. Then you can point out that since the potential energy of the system remains constant because one ball always rises as much as the other descends, this has to be a case of neutral equilibrium. I do not recommend a force analysis to prove the point as it is a long and tedious exercise.

Finally, it is worthwhile to point out that this is the linkage used in the base of the laboratory platform balance so that comparison of the masses will not be biased by position of the individual masses on the platforms.