Instructional Photos

Tidal Forces

High and low tide occur every 12 hours due to forces exerted by the moon on the earth and the oceans waters.

The earth and thw moon together form a system that is held together by a gravitational force of attraction.

The earth/moon system revolves around a common center which is 3/4 of the earth's radius from the center of the earth.

Just as a child on a marry go round experiences a centrifugal force, an observer in the earth/moon system experiences a force outward.The further from the center the greater the force becomes. The observer also experiences a force due to the graviational pull of the moon, closer to the moon the force increases.To see the combined effect of centrifugal and gravitational force, we add the forces together.

Center of the earth, the centrifugal is balances by the gravitational force. The parts of the ocean are the same disatnce from the moon as the center of the earth. These parts are also in balance and experience no tidal forces.

On the side of the earth facing the moon, the net force is toward the moon. This explains why high tide occurs on the earths side nearest the moon.

At points on the earth's surface opposite the moon, the net force is away from the moon. This is why we have a high tide on the far side of the earth.

As the earth spins on it's axis, people at any given location see two high tides each day.