Foucault’s pendulum demonstrates the Earth’s rotation by swinging back and forth in a vertical plane. The longer the string, the slower the swing, and the Earth can be seen rotating from below. The experiment is most effective at the equator or the poles. Today, thousands of Foucault pendulums exist in universities and scientific institutions, but they eventually stop due to energy loss. Originally, the pendulum was started by lighting the rope on fire, but modern ones use magnetic devices.
A Foucault pendulum, like other pendulums, consists of a weight hanging from a rope or thread. The weight is lifted, then released, and the pendulum swings back and forth in the vertical plane, until friction with the attachment point above, or the air around it, slows it down. So the pendulum, like a seesaw, needs another push to get it going again. Sometime before the mid-19th century, physicist Leon Foucault realized that if he could extend the time a pendulum swings before it stops and remove the friction from its attachment above, the Earth could be seen rotating from below. of it, thus demonstrating that it was the Earth rotating under the sky and not vice versa.
Foucault knew that the longer the string attached to the weight, the slower the swing of his pendulum would be. He could hang the string above a nearly frictionless pivot, and if he used a very large weight, the pendulum would swing so slowly that it would appear to change direction as the Earth moved out from beneath it. The downside was that since Foucault’s pendulum was the largest pendulum ever created, Foucault needed a building with a very high ceiling. The Pantheon, a gigantic mausoleum with a pillared entrance and high domed ceiling, was offered, and Foucault’s pendulum had a home.
Foucault’s pendulum not only demonstrates that the Earth rotates on its axis, but like a gyroscope, a free-spinning machine that doesn’t change direction unless pushed, it illustrates Newton’s first law of motion. The reason the Earth can come out from under the pendulum is that the pendulum is set in motion and will not change its motion to match that of the Earth, unless an additional force is acted on it to do so. There was a problem, though, with the proof. Foucault’s pendulum at the Pantheon in Paris, France took more than thirty hours to reach its original position, longer than the time it takes for the Earth to complete one rotation.
A simple equation tells researchers the latitude needed for Foucault’s pendulum to be most effective, and the equation predicts that, at the equator, the Earth will not rotate out from under the pendulum. If Leon Foucault had done his experiment at the equator, he would have seen no change in the direction of the pendulum, only a back and forth motion. The north and south poles are also ideal places for the experiment. At the North Pole, as if suspended from the North Star, Foucault’s pendulum would allow the Earth to rotate out from under it in exactly 24 hours. Videos are available online to illustrate the gradations of motion of the Foucault pendulum between the equator and the poles.
Today there are thousands of Foucault pendulums housed mainly in universities and scientific institutions around the world. These pendulums will not be in perpetual motion, as due to the loss of energy, the weight will rise to lower and lower levels until it points downwards towards the center of the Earth and stops. To start the pendulum, originally, a rope was used to lift the weight up to the maximum height. The rope was then set on fire, so that when it burned, no additional forces affected the motion of the pendulum when it started. Modern pendulums usually use magnetic devices to start and keep the pendulum moving.
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