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Homework Help: Tides and centrifugal force help

  1. Nov 19, 2004 #1
    Ok so I'm reading up on some physical (no that's probably not the right word, enlighten me!) phenomenas in this case the tides.

    So as far as I've understood the water on the surface towards the moon is affected both by earth's gravity (which is bigger) and the moon's, and therefore positions itself in between them (a bit above earth's surface).

    What I don't understand is the similar occurance at the surface away from the moon where a similar high tide is caused. From the explanation I could get of the web I got that there is a centrifugal force acting outwards from the earth's center which is bigger than the moon's gravity and therefore there is a net force acting out from earth's center gathering the water.

    I can't grasp how the tides are supposed to be similar (which the article states, they are equal) since the centrifugal force should be added to the moon's gravity on one side and subtracted on the other.
    Next, what is a centrifugal force, we recently spent 2 lessons discarding our believes that a person in a loop (rollercoaster) is effected by an outward force. Now I am being told that the earth is experiencing an outward force?

    The article's explanation was. If you look at it in a frame of reference where the earth moves naturally around the moon without an inward net force (uuhh what?) a net force would be outwards coming from the counterforce on the centripetal force, newton's third law. (This probably doesn't make any sense but you might understand what I'm trying to say). Either way I don't get it, could someone explain?
  2. jcsd
  3. Nov 19, 2004 #2
    What you are saying makes some sense but it is not the cause of the tides. consider this crude drawing
    M WEW
    this is the moon on the left the earth on the right and water surrounding it.

    now cosider the magnitude of the gravitation force of the moon on all three of these things i.e. the earth and two waters.

    <<<W <<E <W

    the water on the moon side get the strongest pull towards the moon. then the earth as a whole get a less strong pull and then the water on the far side of the earth get the least pull because it is farther away from the moon.
    So lets just say for simplicity that the pull of the moon pulls the close water 3 meters towards it, it pulls the earth 2 meters towards it and it pulls the far water 1 meter towards it.
    again if we refer to the diagram, compared to the earth the water close to the moon is getting pulled 1 meter towards the moon, compared to the earth, and the water on the far side is getting pushed away from the earth because the moon is pulling the earth more than the far side water.
    <W E W>

    I tried to make it as clear a possible. hopefully you understand
  4. Nov 19, 2004 #3
    Ok it sounds quite resonable but for what reason would other explanations I've seen include centrifugal forces?

    One more thing, In the WE situation the earths gravitation is counteracting the moons gravitation while in the other it's adding to the moon's pull, how can the tides be equal?
  5. Nov 19, 2004 #4
    the earths gravitation would not affect the tides because the earth pull evenly on water on both sides cancelling it out. you cant include the earths pull because it doesn't actually cause the water to move. I see what you mean that the earths & moon's pulls should be more but I don't believe that is the case. Sorry, but I am having a hard time putting this into words.

    would you have a link to these sources that involve centripedal (see as centrifugal is only a ficticious force) force?
  6. Nov 19, 2004 #5
    this wasn't as in depth as another one I found about the frame of reference but it should be enough, I'll try to find the other one too
  7. Nov 20, 2004 #6
    May I point out this from your webpage. I would be highly skeptical considering the source. here is where I learned of the reason for tides. It was written by a Phd astronomer who has written many several papers and also a book.
  8. Nov 20, 2004 #7
    http://www.coastalresearch.nl/tides.htm [Broken]

    The interesting thing with this article is that herer the cause of the second tide away from the moon is caused by the rotation of the moon/earth as opposed to your theory (which I have also seen in other articles) it is caused simply by gravity...
    Elaborate :P
    Last edited by a moderator: May 1, 2017
  9. May 21, 2010 #8
    Re: Tides

    I have a problem in solving the shallow water equations using the differences identified in Matlab for the purpose of the work of a predictive model and tidal in the Arabian Gulf Is there anyone who can help me please help and guide everyone who has information
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