Motion and Its Causes

Force and Newton’s Laws of Motion

How does gravity cause tides?

As someone who lives near an ocean will know, there are two high tides and two low tides each day. They’re not at the same time each day, but depend on the phase of the moon. The heights of the tides vary over the seasons as well. What causes the tides?

The moon’s gravitational field exerts a force on the water. Because the field and force depend on distance, the force on the water closest to the moon is strongest. The force on Earth is weaker, and the force on the water on the far side of Earth is smallest. So the water nearest the moon is pulled toward it, and Earth is pulled toward the moon more strongly than the water farthest from the moon. For that reason there is a tidal bulge in the water near the moon, and another on the far side. The bulges, which are the high tides, are not directly under the moon because Earth’s rotation drags the water along with it. Because the day is shorter than the lunar month, the high tides actually come about two hours before the moon is overhead.

Tides also vary greatly with location. The largest tidal variations occur in the Bay of Fundy between the Canadian provinces of Nova Scotia and New Brunswick, where the largest recorded range was 17 meters, or almost 56 feet. There have been many proposals to put a dam across the bay and use the tides to generate electricity, but environmental concerns have blocked construction in that bay.

The sun also affects the tides, but much less than the moon. When the sun, Earth, and the moon are aligned, which happens at both full and new moons, the tides are especially high.


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