THE CAVENDISH EXPERIMENT

A key feature of Newton’s law of gravitation is that all objects attract each other via gravity. Yet in practice, the only gravitational force we ever notice is the force of attraction to the earth. What about the gravitational force between two students sitting beside each other, or between your two fists when you hold them close to each other? The reason that you do not notice these forces is that the gravitational force is incredibly weak, weak compared to other forces that hold you, trees, and rocks together. Gravity is so weak that you would never notice it except for the fact that you are on top of a huge hunk of matter called the earth. The earth mass is so great that, even with the weakness of gravity, the resulting force between you and the earth is big enough to hold you down to the surface.
The gravitational force between two reasonably sized objects is not so small that it cannot be detected, it just requires a very careful experiment that was first performed by Henry Cavendish in 1798. In the Cavendish experiment, two small lead balls are mounted on the end of a light rod. This rod is then suspended on a fine glass fiber as shown in Figure (1a).
As seen in the top view in Figure (1b), two large lead balls are placed near the small ones in such a way that the gravitational force between each pair of large and small balls will cause the rod to rotate in one direction.
Once the rod has settled down, the large lead balls are moved to the position shown in Figure (1c). Now the gravitational force causes the rod to rotate the other way. By measuring the angle that the rod rotates, and by measuring what force is required to rotate the rod by this angle, one can experimentally determine the strength of the gravitational force between the balls. Then by using Newton’s law of gravity

Figure 1: The Cavendish experiment. By moving the large lead balls, the small lead balls are first pulled one way, then the other. By measuring the angle the stick holding the small balls is rotated, one can determine the gravitational force .