Foucault's Pendulum: California Academy of Sciences

Foucault's Pendulum was designed in the nineteeth century. It consists of a massive, long pendulum that swings back and forth in the absence of any external forces. One characteristic of pendulums is that they always swing in the same direction. A pendulum that begins to swing over a straight line, for example, will keep swinging over that line even if the top of the pendulum is rotated or moved. This observation can be explained by Newton's First Law, the law of inertia, which states that a moving object will continue on the same path until it is acted upon by an outside force.

The purpose of Foucault's Pendulum is to prove that the Earth rotates. Were the Earth completely stationary in space, the pendulum's natural tendency to maintain the exact same orientation in space would cause it to remain on the same path for every swing. However, the rotation of the Earth actually causes the area under the pendulum to rotate slightly. Therefore, the pendulum's path relative to the circle of pegs will change very slightly with each swing. As the Earth rotates more, the pendulum will actually knock over all of the pegs in the circle.

The pendulum in the California Academy of Sciences is equipped with an electromagnet that supplies a force on the pendulum that is equal to the force of friction, thus allowing the pendulum to swing infinitely. After a period of 39 hours, every peg in the circle will be knocked down.

I'd learned about pendulums before in my physics class, and I thought that I had a pretty good understanding of them. I know how to find the period of a pendulum based on its length, or the total energy of a pendulum system. However, even though I could do simple problems with pendulums, I'd never really considered the real-world applications of a pendulum. When I saw Foucault's Pendulum, I was amazed that it was possible to use something as mundane as a pendulum, something that I had studied and that I understood, to prove something as important as the fact that the Earth rotates.