photo by "lost coyote," courtesy of Philip Greenspun; image source

Relative Motion & Frame of Reference

This photo (high-res version) reveals that the stars in the night sky seem to spin around in one direction in space. However, as any textbook will tell you, Earth does the spinning which can be shown in the lab by the rotation of the plane of swing of the Foucault pendulum. But the 19th-century philosopher-physicist Ernst Mach didn't buy this--he believed that the existence of relative motion meant that there was no preferred frame of reference. Mach's ideas--see "Mach's principle"--were part of the inspiration for Einstein's general theory of relativity.

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Hyperphysics: Pendulum

You can make a simple pendulum with string or thread, washers, and a paper clip. Time how long it takes the pendulum to go back and forth--that's called the pendulum's period. Try doing this several times and averaging the results. Change the length of the string and the number of washers and see what happens. To learn more, you can visit Hyperphysics: Pendulum.

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image credit: R. Baker, J. Bird, M. Town; larger image

The Foucault Pendulum

The Foucault pendulum demonstrates Earth's rotation by the rotation of the pendulum's plane of swing.  To learn about Foucault's discovery, see this APS News article. The photo above shows a group that set up a Foucault pendulum at the South Pole.

Inertial frames and Newtonian mechanics

Visit Inertial frames and Newtonian mechanics, from the University of New South Wales, for a description of frames of reference in which Newton's laws cannot account for certain kinds of motion.  Step through the animation provided to see what happens when someone on a merry-go-round throws a ball. Don't miss this movie of the same kind of motion, from the University of Illinois. Also, see this Hyperphysics page on the Coriolis Force.