It started off with mum and I watching the floods brought by Sandy on CNN and mum asked what’s the cause for waves. Well wind caused Sandy’s waves. But, Daily tides, high and low, are caused by the moon.
The accepted theory is the moon exerts a force of gravity on earth as the moon rotates earth (both the moon and earth rotate counter clockwise, but earth’s rotation is much faster). The parts of earth passed by the moon are pulled towards the moon, and when water is pulled, tides are formed! Yayy! Sounds easy right.
But, there comes this picture
– with the theory that the moon pulls the side closest to it the most, pulls the middle of the Earth with a medium force and the side furthest away with the least force. So the bulge of water on the left of the pic is pulled the least, appearing as a bulge as Earth is moved towards the right.
The sun comes into the picture as well, also exerting a pull on water. But, the moon’s pull beats the sun. Though the suns is massive, it’s too far away (F=GMm/r2, Newton’s Law of Gravitation) And if the moon and sun are on opposite directions to Earth, high tides become lower.
But then again, if the moon pulls pulls parts of Earth with different distances from it with different forces, each part would have a different acceleration. F=ma. The parts closest to the moon would accelerate the most (in the pic above, it would be the right end of Earth) and the opposite end (the left), the least. And so, with one end gaining acceleration compared to the other, Earth would be stretched and eventually pulled apart.
So, there has to be a restraining force. Earth’s gravity and its tension? Or a centrifugal force? Now what about a centripetal force?
If you want to end up confused (like myself), I suggest you read:
1. Tides and Centrifugal Forces by Paolo Sirtoli. Lots of physics explained here – with equations.
2. Tidal Forces and the Effects on the Solar System by Richard McDonald. Simpler physics and much easier to understand.
3. Tidal misconceptions by Donald. E Simanek. This is the most comprehensive to me, though let me warn you its quite long.
And then there’s the belief that tides don’t form on the equator. But I’ve seen it for myself, they do form! In fact, they’re semidiurnal tides (two equal high tides and low tides in a day). One high tide on each of the pair of “bulges” created. Check out ocean motion. to find out about types of tides at different latitudes on Earth.
I’ll keep you updated on tides once I piece everything together (and find some missing links, like I’m sure the moon’s orbit around Earth at at about a 5 degree angle to Earth’s ecliptic would have an effect as well). Let me know if you’ve got any ideas of your own!
And mum has two questions of her own:
1. Why isn’t paper pulled towards the moon when its so light.
To me, it is, but the effect is so small that you’ll never realize it.
2. Why do you get the highest tides on the full moon. This, I don’t know. So let me know if you do pleaseee.
ps. Here’s some videos on movements of the moon and of Earth.
Some cool stuff here!