A pendulum is loosely defined as something hanging from a fixed point which, when pulled back and released, is free to swing down by gravity and then out and up because of its inertia, or "tendency to stay in motion."
Foucault noticed that if you set a pendulum to swinging, it would seem to change the direction of it's swing with time. But he realized that it was actually the floor and building rotating beneath the pendulum. At last, visible proof of the earth's rotation!
Foucault presented his pendulum to the world at the Pantheon in Paris, where he used a 61 pound bob, and a 220 foot cable to suspend it. The longer cable and heavier bob allowed the pendulum to swing slower and with less slow-down from air friction. He attached a pin to the bottom of the bob, which drew the path of the swinging bob in a ring of wet sand on the floor. Over a period of 24 hours, Foucault demonstrated that the plane of rotation, (the floor) rotated about 270 degrees, demonstrating the rotation of the Earth.
-- HOW DO WE KNOW IT IS THE EARTH ROTATING?
Let's consider the forces acting on the pendulum:
1. Its INERTIA, makes it swing straight out
2. GRAVITY pulls it straight back(actually gravity
pulls it DOWN, the tension in the cable causes
it to swing in an arc.
3. AIR RESISTANCE makes the bob swing in shorter arcs,
but arcs that are just as straight.
4. Since the Pendulum is tied to a building, it will move laterally
as the building moves laterally, but because of the way it is mounted,
it will not twist, even if the building does!
Even though the pendulum seems to rotate with respect to the floor, we know that there is no force to make it rotate. Therfore the floor must be rotating, and the floor is rigidly attached to the Earth, so it must be the
EARTH that is rotating!
Why doesn't it rotate 360 degrees in 24 hours, if the earth does?
Why doesn't the pendulum seem to rotate 360 degrees?
First off, remember that the pendulum only demonstrates rotational motion about the earth's axis. If the pendulum were built directly over the North Pole, it would seem to rotate 360 degrees every 24 hours. However, at the equator, a pendulum would not seem to rotate at all. That is because it is only undergoing lateral motion.
So what about places in between?
The rotation demonstrated by the pendulum can be predicted by the following formula: n=360sinF, where F is the angle of a place from the equator.
For example, in Paris the latitude is about 48.6 degrees, so Foucault's
pendulum "rotated" about 270 degrees.
In Atlanta, the latitude is about 33.75 degrees, so a pendulum
would seem to rotate about 200 degrees in 24 hours.
http://helix.gatech.edu/Classes/ME22...definition.htm
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