Physics Photo of the Week

May 1, 2009

The Coriolis Effect
Students 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 force.

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 first video.

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 future.



Physics Photo of the Week is 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 submit digital (or film) photographs for publication and explanation.  Atmospheric phenomena are especially welcome.  Please send any photos to dcollins@warren-wilson.edu. 

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