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
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 firstname.lastname@example.org.
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.