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Thread: The proof is in the soup : Moon & tides : how ?

  1. #1 The proof is in the soup : Moon & tides : how ? 
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    Here's a nice plate of soup. It represents an ocean on earth.


    Can somebody tell me how to pull out some soup from the plate (moon attraction),
    and make the soup level at the inside plate edge go UP (flood > water going up the shore) ?





    I believe this :

    If the water is pulled, then the waterlevel at the shore will go down.
    This is because you then take out, or slightly pull up an amount of water from the bassin, so the water level drops.

    Unless you could pull the entire amount , the whole ocean, up from the bottom of the sea,
    then indeed the water level at the shore would go up.
    But that seems very unlikely given the gravity force of the earth which keeps the ocean tightly down,
    and the weak attraction force of the moon, especially from such distance.


    Furhter, at the same time on the other side of the earth , there is also a flood effect.
    This is supposedly due to the fact that the earth is moving slightly towards the moon because of it's attraction.
    And the water then stays 'hovering on the spot' for a flood effect ?
    So here the entire massive earth is slightly pulled towards the moon but the water isn't ?

    And further from that point on : If indeed then the earth moves slightly towards the moon, wouldn't it counter the simultanious flood on that side ? (water went up , and now the earth also > zero effect, no ?)



    >> But let's focus first on my first question :

    How to pull out water and make the water level at the shore go UP ?

    The only answer i can find is that the water would be an slightly elastic substance,
    so you could stretch it as you pull, but that's not what i learned in school, don't really believe that.





    What is your oppinion ? Am i missing something here ?


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  3. #2  
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    The one thing the ocean isn't is a bowl of soup, or even a bathtub.

    This is one of my favourite videos ever. It's about sea level rise but lots of good, and surprising, stuff about how the level differs around the world and in different circumstances. Along with all the gravitational and friction effects of tides and other water movements.

    One of the best half hours you'll ever spend. Jerry Mitrovica, Harvard University - YouTube


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  4. #3  
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    Quote Originally Posted by Noa Drake View Post
    Can somebody tell me how to pull out some soup from the plate (moon attraction),
    and make the soup level at the inside plate edge go UP (flood > water going up the shore) ?
    Easy - tilt the plate. This simulates tides. Another way to do it would be to keep the plate level and then put a moon-sized mass next to it; the gravitational attraction of the mass will then attract the soup and the effect will be the same - it will be as if you have tilted the plate.

    But that seems very unlikely given the gravity force of the earth which keeps the ocean tightly down, and the weak attraction force of the moon, especially from such distance.
    Keep in mind that it IS incredibly weak. Over tens of thousands of miles of ocean, the lunar tides are only a few feet! If your soup bowl was six inches across the moon would cause a rise on one side of .0000002 inches.



    So here the entire massive earth is slightly pulled towards the moon but the water isn't ?
    The entire Earth is pulled but it's not very elastic; water is. So it moves more than the Earth does.
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  5. #4  
    Brassica oleracea Strange's Avatar
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    Quote Originally Posted by Noa Drake View Post
    If the water is pulled, then the waterlevel at the shore will go down.
    I'm not quite sure what your mental model is here.

    But what I think you are talking about is the case where the moon is directly above your bowl of soup (or ocean, if you have one handy).

    In this case, then yes, the water would be pulled up at the center of the ocean and would fall at the coasts (assuming they are far enough away). In other words, low tide. When the moon is somewhere over your local coast then the water will be pulled towards the edge of the "bowl" and you will get high tide.


    Tide - Wikipedia, the free encyclopedia

    Am i missing something here ?
    Hard to say. This was all analysed and explained in great detail by a Mr Newton, quite a few years ago.
    ei incumbit probatio qui dicit, non qui negat
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  6. #5  
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    Quote Originally Posted by Noa Drake View Post
    Here's a nice plate of soup. It represents an ocean on earth.


    Can somebody tell me how to pull out some soup from the plate (moon attraction),
    and make the soup level at the inside plate edge go UP (flood > water going up the shore) ?





    I believe this :

    If the water is pulled, then the waterlevel at the shore will go down.
    This is because you then take out, or slightly pull up an amount of water from the bassin, so the water level drops.

    Unless you could pull the entire amount , the whole ocean, up from the bottom of the sea,
    then indeed the water level at the shore would go up.
    But that seems very unlikely given the gravity force of the earth which keeps the ocean tightly down,
    and the weak attraction force of the moon, especially from such distance.


    Furhter, at the same time on the other side of the earth , there is also a flood effect.
    This is supposedly due to the fact that the earth is moving slightly towards the moon because of it's attraction.
    And the water then stays 'hovering on the spot' for a flood effect ?
    So here the entire massive earth is slightly pulled towards the moon but the water isn't ?

    And further from that point on : If indeed then the earth moves slightly towards the moon, wouldn't it counter the simultanious flood on that side ? (water went up , and now the earth also > zero effect, no ?)



    >> But let's focus first on my first question :

    How to pull out water and make the water level at the shore go UP ?

    The only answer i can find is that the water would be an slightly elastic substance,
    so you could stretch it as you pull, but that's not what i learned in school, don't really believe that.





    What is your oppinion ? Am i missing something here ?
    You have to consider what causes tides. The effect is twofold: One is due to the differential of the Moon's gravity across the Earth; its pull is weaker on the far side than it is on the near side. The is due to the Moon's gravity pulling inward on the sides of the Earth ( look at the image Strange provided, the pull from the Moon on the two sides of the Earth is angled towards the center line connecting the two. Now while water isn't elastic, it is very flexible. So the effect is like taking a water balloon (here the balloon itself acts like the Earth's gravity trying to hold the water in a sphere), and pulling it from two ends while simultaneously squeezing the middle. The balloon will distort its shape. The total volume of water doesn't change, but its arrangement does.

    Now the Earth itself also feels these stretching and squeezing forces, however it is much more rigid than water is and holds much closer to original shape.

    Another thing to keep in mind is that, in reality, we are talking about a very small bulge. If there were no continents, the maximum height of the bulge would be less than 1 meter. It is the "sloshing" effect caused by the variation in ocean depth that causes the shoreline tide of several feet that we see.
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