Physics Photo of the Week
The Coriolis Effect
in Earth, Light, and Sky photographed this disk sliding over a level
table. The disk contains a battery-powered fan that levitates the
disk as a hovercraft. Thus the disk can glide over a level
surface with very little friction.
Notice that the disk does not follow a straight path. It starts
out following the broad line in the middle of the table, but it is
deflected towards the top of the photo. There seems to be a force
pulling it towards the top of the photo.
The table is not stationary, but it is being rotated by students on the
perimeter. The video camera is mounted on a superstructure that
is attached to and rotates with the table, so the table looks
stationary. Careful examination of
the weak reflections of the ceiling lights reveals that the table is
rotating underneath the lights. When an object is moving on a
rotating surface, it experiences this strange force called the Coriolis
The Coriolis force is extremely important in the motion of air currents
on the Earth. Storms that bring much rain and wind are cyclones
rotating around a low pressure center. Hurricanes and typhoons
are massive tropical cyclones. As air flows from all directions
to replace the rising air at a low pressure center, the air is
deflected to its right in the northern hemisphere by the same Coriolis
force. The result is
a counter-clockwise rotation in the northern hemisphere.
Back to the table and the frictionless glider.
Another experiment was made by the Earth, Light, and Sky class using
the same rotating table and glider. This time the camera is
mounted on the stationary ceiling, looking down at the table that
rotates beneath it. In the video at right, it can be seen from
the traces that the glider follows a straight path relative to the
fixed laboratory. The table rotates underneath it and doesn't
affect the glider. You can also see how the rotation is
maintained by students around the perimeter. The rods and clamps
above the table in the second clip had supported the camera for the
The motion of the glider in the second clip follows Newton's law of
inertia. There are no forces, so the massive glider continues its
motion in a straight line. However, the first clip shows the
Coriolis force; the glider's
path is not straight. How can a "force" appear out of
nowhere? The answer can be explained by the fact that the table
and frame of reference in the first clip is not an "inertial" frame,
but is rotating. The Coriolis force is an artifact of rotating
frames of reference.
If the table was rotated in the opporite direction (clockwise rather
than counterclockwise), the Coriolis effect will be in the opposite
direction. On Earth, the northern hemisphere acts as a
counterclockwise rotating table. In the southern hemisphere,
looking down on the south pole, the surface of the Earth rotates
clockwise. As a result cyclones rotate counterclockwise in the
northern hemisphere and clockwise in the southern hemisphere.
Watch for another video clip showing rotating cyclones in the near
Photo of the
published weekly during the academic year on Fridays by the Warren
Wilson College Physics
Department. These photos feature interesting phenomena in
the world around us. Students, faculty, and others are invited to
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