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Gravity and tides

In many coastal areas, sea levels rise and fall approximately every 12 hours as a result of the interaction with the gravitational field of the moon.

The gravitational force acting on a point on the side of the Earth facing the moon is bigger than the force at the centre of the Earth and, even more so, on the other side of the Earth.

  • On the side facing the Moon, the Moon pulls the water away from the Earth so that it rises.
  • On the side facing away from the moon, the Moon pulls the Earth away from the water so that the water is further away from the Earth's centre.

The gravitational force exerted by the Sun on a unit mass on Earth is much greater than the force exerted by the Moon. But the Sun is much further away.

Because the force decreases in proportion to the square of the distance, this means that the difference in the force acting on opposite sides of the Earth is much smaller.

Force F1 is greater than F2, which is greater than F3. The difference causes the points to move away from each other and we observe this in the form of tides.