Tides, Gravity and stuff (up to IB)

tidesWe don’t get very high tides here in the Gulf. The body – or better – mass of water required to create them simply isn’t big enough.
Explaining one high tide a day is quite easy. Two or even more is a little trickier. First, the easy bit. Newton’s Universal Law of Gravitation  (the detailed handout called Gravitation IB v2  is not for the faint of heart) says that every body in the Universe attracts every other body in the Universe with a force which is proportional to the product of their masses and inversely proportional to the square of their distances apart. So, the Moon gravitationally pulls or attracts the massive bodies of water in the oceans towards it, with greater force when the bodies are facing it, creating a ‘bulge’ of water. So, big movements of water where there are big oceans next to the coastline.
There’s also a bulge on the other side of the Earth as well, at the same time, which is a bit more subtle. The Earth is more massive than the Moon so they rotate around each other like eccentric dumbbells. The acceleration produced on the further side of the earth is pulling the water inwards while its inertia is tending to keep it where it is, thus forming a bulge. The Sun plays part at different times in the lunar cycle also (see diagram) and coastal geography means that some places on Earth have up to four high and low tides a day, or even none….