The Foucault Pendulum is named for French physicist Jean Foucault who
first used it in 1851 to demonstrate the rotation of the Earth. It was
the first satisfactory demonstration of the Earth's rotation using
laboratory apparatus rather than astronomical observations.
Is the pendulum rotating or is the Earth rotating the ground underneath the pendulum?
If you start a Foucault pendulum swinging in one direction, after a few hours you will notice that it is swinging in a different direction. How does this happen?
Imagine you are in a museum located at the north pole and that the museum has a
Foucault Pendulum suspended from the ceiling at a point exactly over the pole. When you
set the pendulum swinging it will continue to swing in the same direction unless it is
pushed or pulled in some other direction. (This is due to a basic law of nature called
Newton's First Law.) The earth, on the other hand, will rotate once every 24 hours
underneath the pendulum. Thus if you stood watching the pendulum, after a quarter of an
hour or so, you would be likely to notice that the line of the pendulum's swing has
changed to a different direction. This would be especially clear if one marked the
position of the line of swing in the morning and had the pendulum knocking down pegs
arranged in a ring at the center.
However, if you are standing on the floor of a building housing a pendulum (which is
connected to the earth), you will naturally think that the floor is stable and the
pendulum is moving. This is because we naturally assume that the base on which we stand is
stable unless our eyes or sense of balance tells us otherwise. If our base moves slowly or
accelerates smoothly, we are easily fooled into thinking that another object we see is
moving. You have probably experienced this in a car, a train, or an airplane, that begins
to move very slowly and smoothly, and for a split second you think that a nearby car,
train, or even a building, seems to move. Thus, after thinking for a while about the total
situation you might be willing to agree that what you are seeing is a real demonstration
that the earth is rotating under the pendulum and that the line of swing of the pendulum
just appears to rotate.
At the north pole the apparent rotation would be a full circle of 360
degrees each
24-hour day, or about 15 degrees per hour. This case is fairly simple,
because here the
earth and the pendulum are not exerting much influence on each other.
As you move off the
north pole down to a more southerly point like Washington, for example,
the earth not only
rotates under the pendulum, but it carries Washington, the building,
and the pendulum, in
a great circle about its axis. That is, the motion of the earth is now
mixed in a
complicated way with the motion of the pendulum. As you can prove if
you watch the pendulum for a while, the effect of this is to slow down
the apparent rotation of the swing. Instead of seeming to rotate 15
degrees (about 1/24 of a full circle) in one hour,
it only changes by about 9 degrees (about 1/40 of a full circle). The
further south you
go, the slower the apparent rotation gets, and at the equator there is
no rotation at all.
Below the equator the apparent rotation begins again, but in the
opposite direction.
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