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. But 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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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.