This image shows the Earth and the moon, and the bulges produced on each side from the tides.
Click on image for full size
Tidal Forces
The force of gravity caused by an object gets weaker as you move
farther away from that object. In this picture, the Earth is pulling
on the Moon, and the Moon is pulling on the Earth. The Moon pulls
more strongly on the side of the Earth facing the Moon than on the
side facing away from the Moon. Because the gravitational force on one
side of the planet is different from that on the other side, it is called a
tidal force.
Because planets are not perfectly rigid, they change their shape
when they are pulled this way. They get pulled out as if they are
being pushed from the top and bottom (Earth also bulges at the equator all the way around because it is spinning). Thus they form two bulges on either side of the planet. These two bulges are called tides.
On Earth, near the ocean, these tides can actually be seen. The ocean
water rises high along the beach, twice each day.
If a body is very rigid or is not held together well, instead of
getting pushed and pulled out of shape, the tidal forces can actually
tear the body in half, as with comet
Shoemaker-Levy 9.
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