-
Asked by lexie2610 to Chris, Josh, Rebecca, Rob, Susan on 19 Jun 2015.
-
Rebecca Dewey answered on 19 Jun 2015:
The moon is just a big chunk of rock that is travelling through space. Because the Earth is an even bigger chunk of rock, the moon has been caught in its gravitational field and so now the moon orbits the Earth, completing a loop once every 28 days. But just in the same way that the moon is attracted to the Earth, the Earth is also attracted to the moon. That means that the earth follows a very slightly different path through space than it would if we didn’t have a moon (but this difference is pretty small because the moon is a lot smaller than the earth). However, the water on Earth is a liquid and is much easier to influence than the solid rock. The water is attracted to the moon and gets pulled around by the moon’s gravitational field. This also happens in a pattern that repeats every 28 days, as it does the same thing each time the moon passes by a particular place.
-
-
Susan Cartwright answered on 19 Jun 2015:
Gravity.
Newton’s laws tell us that force is mass times acceleration, so the Earth accelerates towards the Moon, pulled by the Moon’s gravity. But Newton’s law of gravity tells us that the force of gravity depends on 1/r squared, where r is the distance from the gravitating object. So the side of the Earth closest to the Moon is feeling a stronger force, and wants to accelerate towards the Moon faster, whereas the side of the Earth furthest away is feeling less force, and wants to accelerate more slowly. The result is that you get two bulges of water, one on the side nearest the Moon (accelerating faster) and one on the side furthest away (being left behind). As the Earth spins, these two bulges move sideways relative to the surface of the Earth, leading to two tides every day.
The Sun also causes tides, but they are smaller – although the Sun’s gravity is greater than the Moon’s, we are much further away, so the relative difference between the near and far sides of the Earth is not as big. But this is why the height of tides varies over a month: spring (higher) tides are when the Moon’s water bulge and the Sun’s line up, at new or full Moon, and neap tides are when they are 90 degrees apart, and first and last quarter Moon. (Note that “spring” tides come from “spring” meaning “jump” – they occur all year round, not just in spring!)
Comments