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If you leave salad dressing for a while, the
water and oil separate, and a clear boundary is
formed between the two. If you then slowly rock the
bottle, waves appear on the boundary. In the same
fashion as the waves on the surface, these waves are
caused by the restoring force of gravity. The
waves on the water's surface occur at the boundary of
air and water, however no matter what the two
substances are, the principle behind waves formed at
the boundary of two substances with differing
densities is the same. When the gap in density
between the two substances gets smaller, restitutive
force decreases, and the period of the wave
increases. If you get a cycle as long as the rotation
of the earth,
a wave totally different
to that of the salad dressing bottle emerges.
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