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# Gravity and the Tides

There are few things in nature as reliable as the tides—the periodic rise and fall of ocean waters. Though tides occur in oceans on Earth, they are controlled by the gravitational influence of the heavenly bodies.

This may seem mysterious—if we fill a bucket with water, we don't expect its level to vary during the day. After all, where would it go?

In this quiz, we'll figure out where the water is going, and what forces drive its ceaseless cycle.

What could be the cause of tides in the ocean?

Suppose there are three identical asteroids next to the Moon as shown in the diagram. If all the asteroids are free to move, what will happen to the distance between asteroids 1 and 2?

Details: ignore the gravitational interaction between the asteroids.

How will asteroids 1 and 3 move with respect to asteroid 2?

Details: ignore the gravitational interaction between the asteroids.

Suppose that at a given time, some location on Earth is in high tide. Must the opposing point on Earth also be in high tide?

Details

• For the purpose of this question, assume that the Earth is fully covered with water.

Tides are determined by the locations of the Sun and the Moon. We can explore a few interesting cases. The first is the New Moon arrangement, in which the Moon is between the Sun and Earth. In this state, the Moon is barely visible at night.

The second case is of Full Moon. In this case the Earth is between the Sun and the Moon.

In both of these cases the Sun, the Earth, and the Moon are collinear, and the tides that are generated are called Spring tides.

Our last case is when the Moon, the Earth, and the Sun form a right triangle. The tides in this position are called the Neap tides.

When the Moon, the Earth and the Sun become collinear, which of the following is true?

Suppose the Earth, the Moon, and the Sun are colinear. Are tides higher at Full or New Moon?

When the Sun, the Earth and the Moon form a right angled triangle, which of the following is true?

As we saw, tides arise due to one simple reason—the gravitational pull from an external body upon the near side of a planet is stronger than the pull upon the far side. This causes the oceans to distort at a global scale, as water is pulled up and away from the Earth in two locations.

Some other instances of tidal forces include the stretching of objects in the highly non-uniform gravitational field of a black hole, or the radially varying forces in a spinning object. Tidal forces are just one example, but the spatial variation of gravitational fields is fundamental to our understanding of cosmology and is the principal question addressed by general relativity.

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