Lecture 7 H. Tides
One triumph of Newton's theory of gravitation was its explanation of the tides:
- the Moon's gravity pulls on the Earth and ocean water, causing the latter to flow
- in places this water piles up leading to higher water levels = "high tide"
- in other places it has flowed away, leading to dimished levels = "low tide"
Q: If the pull of the Moon causes tides, and the Earth rotates under the
Moon about once a day, why are there two tides?
- A: 'cause gravity gets weaker with distance!
- any time forces on an extended body differ in strength across the body, the term "tidal
forces" is used -- we'll see this again when we get to black holes!
Sun also exerts a small tidal pull
- Q: If the pull of Sun's gravity is stronger than the Moon's (we orbit the Sun, not the
Moon!), why does the Sun not cause stronger tides?
- A: Because it's the difference in the pull of gravity that counts, and the difference
is greater for closer objects -- and the Moon is a lot closer!
- Q: What configurations are optimal for high tides?
- A: When the Moon and Sun are opposed or in conjunction! Which lunar phases are these?