Physics Photo of the Week

October 18, 2013

Carnival Ride
Look through the colors and the shadows and you see a pair of whirling pendulums hung from a rotating wheel that shows a laboratory version of a common carnival ride.  The main wheel is driven by an electric motor (a motorized chemistry stirrer borrowed from the Mars Hill University Chemistry Department).  The spindle on the stirrer has a rubber "shubbing wheel" on the rapidly rotating stirring spindle that snubs against the outside rim of the large wheel.  The two pendulums are 50 g hook weights attached freely to holes in the lower rim of the wheel.  The whole wheel spins at a constant rate, and the pendulums swing out according to the speed of the rotation.  The motion is shown below in slow motion (about 1/4 the actual rate).  The apparatus is part of a physics experiment at Mars Hill University.  Mars Hill physics student Grace Lancaster operated the video camera.  Warren Wilson College Physics and Chemistry Department loaned the video camera that is capable of slow motion photography.

The rainbow sequence of colors, considered either "attractive" or "distracting" illustrate an entirely other physics phenomenon associated with using the overhead video projector as the bright light source.  The projector actually "flickers" at a high rate in different colors that our eyes even-out.  The camera was taking 120 pictures/sec, each picture had a very short shutter opening time ~1/1000 sec.  As a result the camera catches the projector light at different parts of the red-breen-blue cycle of the three primary colors to produce white light. 

The main physics interest in the rotating pendulums is the physics determining the angle that the orbiting pendulums hang from the vertical.  Any object forced to rotate in a circle must experience a force directed toward the center  of the circle.  Without this center-directed force on an orbiting object, the object would merely fly off on a tangent.  The direction of the orbiting objects' velocities is constantly changing.  The laws of physics formulated about 325 years ago by Isaac Newton state that any object whose velocity is changing, even though the velocity changes only in its direction, must be acted upon by some force.  That force on an orbiting object must be directed toward the center of the orbit.

The leaning of a pendulum from the vertical is also analogous to the leaning of a "dangle toy" often hanging from vehicle rear-view mirrors (a distraction from driving...).  When the car accelerates in the forward direction, it must also apply a forward force to the pendulum dangle toy suspended from the mirror.  The angle of the dangle toy from the mirror is a simple "accelerometer".  If the car rounds a curve, the dangle toy leans outward so that the susension points toward the center of the curve.

In the rotating pendulum experiment the angle of the orbiting pendulum is determined by the vector combination of the inward centripetal force (directed horizontally) and the downward gravitational force on the pendulum's center of mass.  The physics students measured the angle and the rate of revolution of the wheel from the digital photographs.  They found that the inward force determined from the geometry of the pendulum's angle matched the inward force from Newton's formula for centripetal force (MV2/R) very well.

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

All photos and discussions are copyright by Donald Collins or by the person credited for the photo and/or discussion.  These photos and discussions may be used for private individual use or educational use.  Any commercial use without written permission of the photoprovider is forbidden.

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