The Foucault pendulum: a simple explanation, some history
about the ideas of inertial frames, some implications, correction of a few common misunderstandings and finally a detailed analysis of the motion.
Foucault Pendulum at the University of New South Wales (animations
pendulum ball (above, left) has just been released from in front
of the camera position. The close-up (middle) shows a fine cable
under the handrail of the stairs. Behind this is a line on the
wall. Together they define a reference plane in the North-South
direction, and the pendulum swings in that plane.
hour later, the amplitude of the swing has diminished, and the
plane has precessed about 9 degrees in the anti-clockwise direction
(Sydney is in the Southern hemisphere*). We now see the pendulum
crossing the reference plane.
Why Does The
Orbit Of Foucault's Pendulum Precess?
put a pendulum above the South Pole and sets it swinging in a simple arc.
To someone directly above the Pole and not turning with the earth, the
pendulum would seem to trace repeatedly an arc in the same plane while
the earth rotated slowly clockwise below it. To someone on the earth, however,
the earth seems to be stationary, and the plane of the pendulum's motion
would seem to move slowly anticlockwise, viewed from above. We say that
the pendulum's motion precesses. The earth turns on its axis every
23.93 hours, so to the terrestrial observer at the pole, the plane of the
pendulum seems to precess through 360 degrees in that time.
this paragraph was written, it was hypothetical. Town et al have since assembled and reported the motion of a pendulum
at the South Pole.
situation is more complicated for other latitudes. On the equator,
the pendulum would not precess at all—its precession period is infinite.
At intermediate latitudes the period has intermediate values. One can
calculate that the precession period for an ideal pendulum and support
system is 23.93 hours divided by the sine of the latitude (see details).
For example, at Sydney's latitude of 34 degrees S, the period is about
43 hours, a precession rate of about one degree every seven minutes, in agreement with the observed motion of our pendulum.
The effect that causes a pendulum to appear to veer slightly to the left
(in the Southern hemisphere) is similar to that responsible for the apparent motion
of the major ocean currents. In the Southern hemisphere these are anticlockwise
(they appear to turn to the left) and conversely in the Northern hemisphere.
choose to say that the earth was stationary but that mysterious forces
make moving objects turn. Using Newton's laws and the known rotation
rate of the earth, one can calculate the size of these "forces", which
are called centrifugal and Coriolis forces, together known as fictitious forces. Because it is so convenient to measure
motion with respect to the surface of the earth, these fictitious forces are often
used in calculation.
below shows a pendulum swinging above the South pole. At left is the
view from a position high above the equator at midday on the equinox—an
observer near the sun would see this. The view at right is the view
from directly below the South pole. Note how the path, as seen from
the Earth, curves always to the left. (To make the details easy to
see, the pendulum is depicted as very large and very slow. the amplitude
of its motion was chosen equal to the radius of the Earth, and its
period 8 hours: these values have no special significance. The animation
makes some approximations about the motion.
non-rotating mean? What is the frame of reference in which centrifugal
and Coriolis forces vanish, the frame where Newton's laws work? Observationally,
we find that this Newtonian or inertial frame is one in which the distant
galaxies are not rotating. But if we removed everything in the universe
except the earth, how would we know if the earth were turning or not?
How would the pendulum know whether to precess or not? Or, to put the
question formally, is it just a coincidence that the frame in which
the distant galaxies do not rotate is an inertial frame? Ernst Mach
thought not, and speculated that the distant stars must somehow affect
inertia (Mach's Principle), but this idea is not widely supported. The cosmological hypothesis of the inflationary
universe offers hope of a different resolution: if the universe expanded
exceedingly rapidly in its early phase, any initial rotation will have
slowed down correspondingly and so the distant objects have almost
no rotation, no matter what their initial condition. For more about inertial frames, see our Relativity
What of Ptolemy's objection?
So, if we are whizzing around the earth's axis, and around the sun. why don't we feel it? The answer is that we do not feel uniform motion: we feel forces, and they are more closely associated with acceleration than with motion. (See An introduction to Galilean and Newtonian mechanics.) If the sea is smooth and the ship's motion also smooth, you may not even notice that your ship is moving, though you will notice the waves if present. The waves accelerate the ship up and down, so it exerts variable forces on your feet. They accelerate you, and you feel the variable forces doing that. The acceleration due to the Earth's rotation, at Sydney's latitude, is 28 mm.s−2. This requires a force that is 0.3% of your weight, and it doesn't vary quickly. From this calculation, you wouldn't expect to feel the Earth rotating. Due to its orbit around the sun, the acceleration is 7 mm.s−2. The speeds may be high, but the accelerations are trivial. In motion, you don't feel speed, you feel the forces associated with acceleration. (See Physclips' section on circular motion for the details of these calculations.)
Does water drain clockwise or anticlockwise?
rotation of the earth affect the way in which water runs down the plug
hole when you empty the bath? Some people say that the water goes down
clockwise in the Southern hemisphere and anticlockwise in the Northern
hemisphere. Such people have probably never, or very rarely, looked.
In some bathtubs (basins, toilets etc) and under some conditions, the
water runs out clockwise, in others it runs out anticlockwise. There
is no correlation with the hemisphere. Other effects may lead
to the direction of draining. For instance, some basins have separate
cold and hot water taps that are positioned symmetrically left and
right. If you fill the basin using the left hand tap, you set up a
rotation in one direction, and this will determine the direction in
which it drains. Using the other tap reverses the direction. Many basins
and baths are sufficiently symmetrical that it is possible, with some
care, to have the water drain with no observable rotation.
of the Earth does, however, account for the direction of the major circulations
of air and wind on the planet. A non-mathematical discussion of the coriolis
forces that give rise to these currents.
The 'plane' of the pendulum's swing is not fixed in space
is worthwhile correcting a common misunderstanding about Foucault's
Pendulum. It is sometimes said (perhaps poetically) that the pendulum swings in
a plane fixed with respect to the distant stars while the Earth rotates beneath it.
This is true at the poles. (It is also true for a pendulum swinging
East-West at the equator.) At all other latitudes, however, it is not true. At all other latitudes, the plane of the pendulum's motion rotates with respect to
an inertial frame.
It is easy to deal with this misunderstanding. Consider a pendulum at the equator, swinging in a North South plane. It's obvious from symmetry that the plane of this pendulum doesn't rotate with respect to the earth and that, relative to an inertial frame, it rotates once every 24 hours.
Alternatively, consider the motion of a point on the earth at a place that is neither at the poles or the equator. During a day, a vertical line at that place traces out a cone, as shown in the sketch at right. (If the earth were not turning, the half angle of the cone would be 90° minus the latitude.) During each cycle of the pendulum, when it reaches its lowest point its supporting wire passes very close to the vertical. So, at each lowest point of the pendulum, its wire is a different line in this cone. This cone is not a plane, so those lines do not all lie in the same plane!
For yet another argument, consider the motion of the pendulum after one rotation of the earth. With respect to the earth, the period of precession of the pendulum is 23.9 hours divided by the sine of the latitude. For most latitudes, this is considerably longer than a day. So, after the earth has turned once, the pendulum has not returned to its original plane with respect to the earth. For example, our pendulum in Sydney precesses at a rate of one degree every seven minutes, or one complete circle in 43 hours.
(I apologise for emphasising this rather obvious point. I only do so because a correspondent has pointed out to me that many web pages about the Foucault pendulum – and even, allegedly, a few old text books! – make the mistake of stating that the pendulum swings in a fixed plane while the earth rotates beneath it.)
So, what is the path of motion of the pendulum? Remember that the point of suspension of
the pendulum is accelerating around Earth's axis. So the forces acting on the pendulum are a little complicated, and to describe its motion requires some mathematics. (Indeed, even talking of a 'plane' of motion on a short time scale is an approximation because even in half a cycle the
supporting wire actually sweeps out a very slightly curved
Foucault Pendulum at the School of Physics of The University
of New South Wales is a "hands-on" version. There
is no electromagnetic drive but, because of its size once it
is started it will swing for several hours. Visitors are invited
to start it swinging in a plane that is accurately defined
by a fixed vertical wire and a vertical line on the wall. (See
the animation above) The pendulum takes seven minutes to
precess one degree, but even smaller angles than this can be
seen by sighting along the reference plane.
Motion of Foucault's Pendulum
more information about the motion of the Foucault pendulum