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Side of tide due to moon and sun's gravitational pull

  1. Jun 6, 2005 #1
    Hi everyone, sorry if I'm new to this and this subject have already been posted, but i have a test tomorrow, and i need help.
    The question is that when the sun, the moon, and the earth are in a straight line, with the moon is in between the sun and the earth. It is reasonable to think that the water would be pulled towards the sun and moon, but in my textbook, a diagram shows the on the other side of the earth, there is also a high tide. Why would the water also bugle out on the other side? What force is acting upon it to make it bugle out?
    PS I asked my science teacher and she told me to wait until gr.11 physics, but the test is tomorrow!! any help would be greatly appreciated.
     
    Last edited: Jun 6, 2005
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  3. Jun 6, 2005 #2

    Danger

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    The gravitational attraction of the sun/moon pulls the Earth and the water equally. Because there is inertial involved, however, the Earth lags a little behind the moon-side water, and the dark-side water lags behind both.

    edit: for clarification; I shouldn't have said that it's equal in that statement, because it sort of gives the wrong impression to what follows. I meant that it's uniform and in accordance with gravitational laws. The 'lag' is because of the inverse-square law, which means that each component of the question (#1 water mass/Earth/#2 water mass) experiences a different amount of pull and reacts accordingly, as opposed to behaving as a unit. I'm sorry that I didn't state it better the first time.
     
    Last edited: Jun 6, 2005
  4. Jun 6, 2005 #3
    If it is lagging behind, shouldn't it be at about 90 degrees to the tide created by the moon? Why would it be directly across from the moon-created tide?

    edit after I saw your: you happened to confuse me even more, i'm only a slightly-above average grade eight, would you be so kind as to explain it in simpler words?
     
    Last edited: Jun 6, 2005
  5. Jun 6, 2005 #4

    Danger

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    Sorry. :redface:
    What you have to do is to think of the three components separately. First you have the water that's on the same side of Earth as the moon. Call that w1. Then you have the Earth itself, which is a little farther away from the moon. Finally, there is the water on the side farthest from the moon (w2).
    I'm going to ignore the sun completely, because the same thing applies with any lunar tide. It doesn't matter where the sun is.
    Gravitational pull decreases as the square of the distance between centres of the 2 masses involved. For the sake of simplicity, let's say that w1 is distance (d) away from the moon. It will feel 1 lunar gravity (lg) of pull. Earth is d+1 away, and so experiences less pull. The far ocean w2 is d+2 away, and so feels the least pull of all.
     
    Last edited: Jun 7, 2005
  6. Jun 6, 2005 #5
    Ok, I understand everything you just said. But if w2 feels the least pull, it is still being pulled toward the moon and the sun(assuming when they are in line and in the same direction) why would it bugle outward instead of flat against the earth?
     
  7. Jun 6, 2005 #6

    Danger

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    I know that it seems a little strange. That's where the inertia part comes in. Everything has a natural resistance to being disturbed from it's place. Neither the water nor the Earth wants to move toward the moon, but the gravity forces them to. Since the Earth is pulled more than the far ocean, that water stays behind a while when the Earth moves away from it. The movement of Earth itself is very minor, and is cancelled out continuously because the moon is pulling in a different direction every second as it orbits us. It's the sheer volume of water being pulled away from shorelines that makes the tides so noticeable even though the water isn't being pulled very far upward.
    I hope this helps. If it's still unclear, I'll keep trying, and maybe try to post a diagram.
     
  8. Jun 6, 2005 #7
    Thanks a lot! I think it's clear enough to get me a mark on teh test if it comes up. I have now received about 5 different answers about this from many different sources. All of them saying something like what you said, but none are as clear, or maybe they're just too bothered to explain it fully. Once again, thank you!
     
  9. Jun 6, 2005 #8

    Danger

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    You're entirely welcome. There aren't an awful lot of situations where I can help, because I'm not all that knowledgeable myself. It's very gratifying when one arises. Good luck on the test. :smile:
     
  10. Jun 6, 2005 #9

    russ_watters

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    Maybe too late, but to put it another way, the high tide on the far side of the earth is caused by the earth being pulled away from the ocean on that side.
     
  11. Jun 6, 2005 #10
    its never too late, you just summarized what Danger said. Thanks.
     
  12. Jun 9, 2005 #11

    Danger

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    So....? How'd it go, bud? This is just a hit-and-run post, because I can't stay, but give us a heads-up on how the test turned out. I'll check tomorrow for results.
     
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