Rm. P203: A class on what tides are and what causes them.

I find a lot of people have a hard time with understanding tides. I asked my younger son (15), who is very good in science, to explain tides and he said they are caused by gravity. I then asked him how that causes the bulge on the opposite side. He did not know and he is not the only one. Tideal forces are not only important for understanding the tides here on Earth, but also important for understanding what happens when an object orbits to close to a very massive object. It explaines why the comet, Shoemaker-Levi, that hit Jupiter became many pieces before it hit Jupiter several times.

In understanding what tides are and how they are caused we need first understand some terms.

Centrifugal force (red arrows) is what tried to throw you off the merry-go-round on the playground as it was spun very fast by the big kids.

Gravity (red arrows) is the force that pulled you down when you slipped off the top rung of the monkey bars. (You can see I had an interesting time at playgrounds when I was a kid). Gravity is what pulls stuff down (it is why there is a down) to the ground.

Gravity hols the Earth together, causes things to fall, and gives us weight.

Without gravity, Centrifugal force would rip Earth apart.

Opposing forces is simply two forces, of any type, that work in opposite directions.

Orbit is when an object goes around another object. The object is falling toward the other object at an angle and always misses hitting that object. Imagine that you are standing on a beach ball in space and that beach ball has the same gravity as you would feel on earth. Now imagine you throw a baseball. Now that baseball will travel straight away from you due to the force of your throw and will curve downward from the force of gravity (in this case the beach balls gravity). As the baseball falls it find no surface to hit because the beach ball you are standing on is to far to one side to hit. So the ball keeps traveling in the direction you threw it plus keeps falling toward the beach ball. Until it comes all the way around, where you have to duck, over and over. That is an orbit (not you ducking but the baseball going around and around the beach ball).

Force of throw (yellow) vs. force of gravity (green) equals path of ball (red) note orbit interrupted by ground!

Force of throw (yellow) vs. force of gravity (green) equals path of ball (red) note orbit

Things in orbit are called Satellites.

Ok, I think those are all the terms that I'll be using that need some explanation. As you can already see this is not going to be some highly scientific explanation, but will instead be very general in nature. You should never-the-less come away with a good accurate understanding of this action that is common, but is often poorly understood.

To understand tides better we will have to look a little closer to an object in orbit. You remember that there are two forces acting on the object that creates it's orbit, centrifugal force and gravity.

These forces are opposing forces and are trying to force the object in two opposite directions at the same time. It is these two forces that are called tidal forces and create tides. Looking at the diagram below: The centrifugal force on the outside of the orbit line pulls stronger on that side than does gravity from the body being orbited. Inside the orbit line gravity from the boby being orbited pulls stronger than centrifugal force. This tug o war causes the bulges we call tides.

About now you may be saying to yourself "Hey! This sounds good, but the Moon orbits around the Earth, not the Earth around the Moon. The Moon does not have tides and the Earth does. What gives?". Well the Moon has tides but they are forever locked in place and are very small because roch does not streach as much as water. The Moons tides are locked in place because the Moon always keeps one side toward us. It is theorized that this was due to the tides slowing the moons rotation to one rotation per orbit (go into the how of that later). The Earth does orbit around the Earth / Moon's Center of gravity as does the moon. The earths orbit looks like - A big wobble. It is this wobble that astronomers look for in stars to detect any planets that may br in orbit around that star.


Two bodys of equal mass in orbit.

Earth / Moon Orbit

Now that you have the basics of tides, what does it mean? You already know tides in the ocean raises and lowers the water level around our shores. As it was already mentioned, trying to detect wobbles in stars is being used to search for other planets outside our Solar System. Tides put slow brakes on rotation. This is what they suspect happened so that the Moon now rotates only fast enough to keep one face always toward Earth

Lunar Tides are rotated out of alignment by the Moons rotation.

As tidal bulge rotates away, Earths gravity trys to pull it back

Over time the rotation is slowed to be in sync with Moon's orbit. Tides are locked and don't move.

Did you know that the Moon is getting further an further away with each orbit and may get so far away that we my lose it. This because of tides too! As we saw previously the Moons rotation was slowed because of Earth pulling back the tidal bulge as it was rotated out of the Earth Moon alignment by the moons rotation. The same thing is happining to the Earth. Our direction of rotation is in the same direction as the Moons direction in orbit. Now as the tidal bulge on Earth is rotated away by Earths Rotation, it out races the Moon. The gravity of all that watter and rock in the bulge is actually pulling the Moon along faster in its orbit. At the same time the Moon is pulling back on Earths tidal bulge and slowing down the Earths rotation. Now as the Moon is being pulled faster in its orbit, this causes the Moon to orbit further away, the faster the further. Its not much but it is measureable.

Moon raises tides.

Tides are rotated past Moon.

Tidal bulge pulls Moon forward in orbit and Moon pulls back on tide slowing Earths rotation.

Earths gravity pulls back on the tides and keeps them from being streched out all the way to the Moon. The Moons gravity does the same thing, but what would happen if the tidal forces over came the force of the bodies own gravity. centrifugal force would sling moon/planet/star/comet/meteor parts in one direction while gravity from the disrupthing body being orbited to closely would pull the rest of the satellite in the other direction. That may be what caused Saturns rings.

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