How do the tides work?

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How do the tides work?
« on: June 19, 2018, 06:08:26 PM »
When you were first taught about tides, you probably picked up something along these lines, "The moon pulls the water up, and the sun does too, but not as much." That sound about like what you remember?

If you stop and think about it, you'll probably figure out that isn't the whole story. That's just as far as we bother to teach in science class. It isn't wrong, it's just woefully incomplete.

The side of the Earth closest to the Moon IS pulled harder by the Moon's gravity than the opposite side of the Earth. That's true, but that difference is really very small. The Moon's maximum tidal force is around 0.0000011 g. That's 0.00011% of normal gravity.

A common question here is to ask, "Why don't I weigh less during high tide?" If you read that number I just quoted, you'll see the answer. You DO weigh less at high tide. You weigh 0.00011% less. If you weigh 150 lbs at low tide, your scale will show 150.000165 lbs at high tide. So you do weigh more, but you're highly unlikely to have the means to measure anything that precisely.

You've probably also noticed that there are tidal bulges on BOTH sides of the globe... not just the one closest to the Moon. "Why is the water on the opposite side pushed away from the Moon?" Remember that the Earth is pulled towards the Moon too. The force pulling on the near-side is greatest, the force on the far side is smallest, and the force on the middle is in between. So the water nearest the Moon is pulled towards the Moon harder than the Earth below it. The water on the far side is pulled towards the Moon less than the Earth below it. Thus, bulges on both sides.

This brings us to something like, "If the force is so small, then how could it possibly raise the water level at all?" This is where the phenomenon is really interesting. If you take that 0.00011% g and try to lift some water with it, the water really isn't going to move at all. Anything less than 1 g won't lift the water off the ground. And in fact, the tides do NOT lift the water off the ground. They raise the water level higher than the level around it. The water level must drop at the low-tide points so that the water can rise at the high-tide points.

Thinking in 3D, we see that the tidal forces do not pull strictly up/down. Between the high and low tide points, the force vectors push in different directions. Here's a good diagram showing that:

What we see is that a huge portion of the ocean is experiencing a sideways force. That sideways force pushes water towards the tidal bulge.

Finally, "Why are tides higher in some places and lower in other places?" Remember that the motion of the water causing these tides is actually sideways. The water is sloshing from the low-tide to the high-tide area. (Ok really the Earth is rotating underneath the bulge more than the bulge is moving, but either way.) As water sloshes sideways, it is driven up onto land features and rises as a result. So the shapes of bays have a dramatic effect on tides.

Here's a great video:

How does any of this work on FE? I don't know that. Ya'll feel free to jump in with that.
« Last Edit: June 19, 2018, 06:14:12 PM by ICanScienceThat »

Re: How do the tides work?
« Reply #1 on: Today at 05:03:04 PM »
When you were first taught about tides, you probably picked up something along these lines, "The moon pulls the water up, and the sun does too, but not as much." That sound about like what you remember?

If you stop and think about it, you'll probably figure out that isn't the whole story. That's just as far as we bother to teach in science class. It isn't wrong, it's just woefully incomplete.

The side of the Earth closest to the Moon IS pulled harder by the Moon's gravity than the opposite side of the Earth. That's true, but that difference is really very small. The Moon's maximum tidal force is around 0.0000011 g. That's 0.00011% of normal gravity.

A common question here is to ask, "Why don't I weigh less during high tide?" If you read that number I just quoted, you'll see the answer. You DO weigh less at high tide. You weigh 0.00011% less. If you weigh 150 lbs at low tide, your scale will show 150.000165 lbs at high tide. So you do weigh more, but you're highly unlikely to have the means to measure anything that precisely.

You've probably also noticed that there are tidal bulges on BOTH sides of the globe... not just the one closest to the Moon. "Why is the water on the opposite side pushed away from the Moon?" Remember that the Earth is pulled towards the Moon too. The force pulling on the near-side is greatest, the force on the far side is smallest, and the force on the middle is in between. So the water nearest the Moon is pulled towards the Moon harder than the Earth below it. The water on the far side is pulled towards the Moon less than the Earth below it. Thus, bulges on both sides.

This brings us to something like, "If the force is so small, then how could it possibly raise the water level at all?" This is where the phenomenon is really interesting. If you take that 0.00011% g and try to lift some water with it, the water really isn't going to move at all. Anything less than 1 g won't lift the water off the ground. And in fact, the tides do NOT lift the water off the ground. They raise the water level higher than the level around it. The water level must drop at the low-tide points so that the water can rise at the high-tide points.

Thinking in 3D, we see that the tidal forces do not pull strictly up/down. Between the high and low tide points, the force vectors push in different directions. Here's a good diagram showing that:

What we see is that a huge portion of the ocean is experiencing a sideways force. That sideways force pushes water towards the tidal bulge.

Finally, "Why are tides higher in some places and lower in other places?" Remember that the motion of the water causing these tides is actually sideways. The water is sloshing from the low-tide to the high-tide area. (Ok really the Earth is rotating underneath the bulge more than the bulge is moving, but either way.) As water sloshes sideways, it is driven up onto land features and rises as a result. So the shapes of bays have a dramatic effect on tides.

Here's a great video:

How does any of this work on FE? I don't know that. Ya'll feel free to jump in with that.

Thanks for the post I am learning about the tides myself as time permits.  I'll need to spend more time comparing my understanding to other explanations but just want to share my own understanding I have developed:

My understanding of how tidal forces (or Tide Generating Forces) work following that very diagram result in the rise of large bodies of water is because these tidal forces are able to move very large bodies of water a very small displacement. So effectively, since the water doesn't compress and has nowhere else to go it piles up in a converging location where we can easily observe "the bulges".  In other words, small tidal forces are able to generate an easily observable effect over large bodies of water given geography that is conducive to the effect accumulating resulting in an easily observable effect.  This means that the bulge is not a result of being pulled up or tugged up from the surface of the earth directly by the moon or sun's gravity as we might have been so poorly taught :)  This seems to explain why smaller bodies of water, land masses, people, and certain geographic features disrupting the cumulative effect over some bodies of water doesn't manifest itself as quite noticeable and measurable changes in height.

Additionally, tides have nothing to do with the spin of earth since tides are due to Tide Generating Forces but this doesn't mean the spinning of the earth doesn't contribute to what is traditionally measured and called "the tide(s)".

 
« Last Edit: Today at 05:09:12 PM by Piesigma »