This exhibit consists of four three foot long dowel rods, each with a one pound weight. On two of the rods, the weight is very near to one end; on another rod is approximately one third of the distance from one end; and on the last it is in the center of the rod.Holding one of the rods vertically, place one end on a finger or palm and try to balance it. Now reverse the rod end for end and try balancing it again.
Imagine at the playground two swings side by side. On one swing is a thirty pound child and on the other swing a three hundred pound adult. Almost everyone would expect that the heavy weight adult would be much harder to move than the light weight child.
Inertia is the tendency of a stationary object to remain stationary and of a moving object to continue moving in the same direction at the same speed. The heavier an object, the more inertia it has. The heavy weight adult in the playground swing would be much harder to move than the light weight child because the adult weighs more, and therefore has more inertia.
In this exhibit, when the visitor tries to balance the rod in his or her hand, the rod is trying to pivot or rotate about its lower end. Hence this exhibit is demonstrating rotational inertia. (An example of straight line or linear inertia would be standing alongside the side wall at an ice rink and trying to push an object across the ice.)
Gravity is always pulling down on the weighted rods. When a visitor tries to balance the rod in his or her hand, the visitor's muscles are supplying the force to oppose gravity. If the rod is perfectly straight up and down (perfectly vertical) the force of gravity pulls straight down along the rod and it remains straight up and down. If the rod is even slightly tilted, the force of gravity is no longer straight down along the rod so the rod begins to pivot about its lower end. The visitor must move his or her hand so that the rod is perfectly straight up and down again. And when the rod is tilted, the inertia of its upper end determines how fast the rod begins pivoting. The greater the rotational inertia the slower the rod begins pivoting and therefore the more time the visitor has to react and keep the rod upright.
There are two ways to change the amount of rotational inertia. The most obvious is to change the amount of the weight. The second way to change the amount of rotational inertia is to change the distance from the weight to the point of rotation (where the lower end of the rod meets the visitor's hand). In this exhibit the amount of the weight cannot be changed but there are five different possible distances between the point of rotation and the weight.