This exhibit consists of a number of solid cylinders and rings or donuts that are raced down a surface with a gentle slope. Any of the cylinders or rings held at the top of the sloped surface will roll downhill when released. This rolling is caused by the force of gravity. Because the cylinder or ring has the potential to begin moving when released, it is said to have potential energy. All of this potential energy is due to gravity and the distance the object can "fall" as it rolls down the slope. Two different objects that have the same weight will have the same potential energy, regardless of the relative shapes of the two objects or how those weights are distributed.
When a cylinder or ring at the top of the slope is released, it begins moving down the slope. Some of the potential energy has been converted to moving or kinetic energy. As the object continues down the slope, more and more of the potential energy is converted to kinetic energy, but the potential energy is not zero until the instant the object can no longer fall because it has reached the lower end of the slope. The total fall of the sloped surface from the upper to lower end is about nine inches. An object has the exact same amount of potential energy due to gravity whether it will free fall a distance of nine inches or roll down a slope with an overall fall of nine inches, because it is undergoing the same change in vertical distance.
In a free fall all of the potential energy due to gravity is converted to free fall or linear kinetic energy and the object develops a maximum falling speed. As compared to a free fall, when an object "falls" by rolling down a slope however much more time is required to drop the same vertical distance. Some of the potential energy must be converted to rotational kinetic energy, used to rotate the object about its center. This leaves less energy available to become linear kinetic energy so more time is needed to roll to the bottom of the slope than would be needed to free fall the same vertical distance.
A stationary object tends to remain stationary because of its inertia. The more an object weighs, the greater its inertia. Energy is required to overcome an object's inertia. The greater its inertia, the more energy required to get it moving. The amount of energy needed to cause an object to rotate depends upon its rotational inertia. Again, the greater an object's rotational inertia, the greater the amount of energy needed to make it rotate.
To simplify the discussion of rotational inertia, consider first a solid cylinder. There are two ways to change the rotational inertia of the cylinder. The most obvious is to hold its size constant but change its weight. As an example, the first cylinder could be made out of plastic and be fairly light, and the second the same size but made out of iron and therefore much heavier. The two cylinders are the same size but the heavier cylinder would have more rotational inertia. The second way to change the rotational inertia of an object is to hold its weight constant but change its diameter.. In this example the first cylinder would again be made out of plastic and the second made out of iron but with a much smaller diameter. The two cylinders are the same weight but the larger diameter cylinder would have more rotational inertia.
Now consider a solid cylinder and a hollow ring. They have the same diameters but the ring is made of a much heavier material so that their weights are the same. Some of the weight of the cylinder is close to the center and thus has a relatively small rotational inertia. In contrast, all of the weight of the ring is at some distance from its center and thus has a large rotational inertia. The net result is that the ring has a much greater total rotational inertia than the cylinder. If the cylinder and ring are placed at the top of the sloped surface of the exhibit they have the exact same potential energy because they have the same weight. However, the ring will require much more rotational kinetic energy than the cylinder, meaning in turn that the cylinder will have much more linear kinetic energy. The cylinder will reach the bottom of the slope before the ring.
The yellow cylinders and rings were created to allow visitors to conduct controlled experiments of their own. All of these objects have a weight of either one or two units. Using the single cylinder and the two cylinders glued together to make a double, the visitor can compare the effect of doubling the weight when the diameter is held constant. These two cylinders roll at essentially the same speed. This shows that when the distribution of weight is identical, doubling the weight does not change the speed. Since the double cylinder has twice the contact area with the surface, these two also show that friction of the rolling object with the surface does not explain the difference in the speed between different objects.
Now compare the single width full diameter yellow cylinder and the double width small diameter yellow cylinder. These two objects have the same weight but different diameters. (CAN THE READER SUGGEST WHY DO THESE TWO OBJECTS ROLL AT THE SAME SPEED?)
Finally compare the double width full diameter yellow ring with the single width full diameter yellow cylinder. These two objects have the same weight and the same diameters. However all of the weight of the ring is at its outer edge, while the weight of the cylinder is evenly distributed from its center to its outer edge. The cylinder has much less rotational inertia and therefore rolls at a much faster speed.