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Gravity between the Earth and the Moon

  1. Jun 8, 2012 #1
    I'm just reading a book and it says:

    '' Humans are too small to be affected noticeably by the tides. The earth has tides because its big, thousands of kilometres across. This gives the gravity from the Moon room to weaken.''

    Why is it that the more massive an object is, the more it is susceptible to the gravity of the moon? Because if the Moon's gravity weakens over the Earths body, that would mean that there is a reduced pull towards the moon, reducing the height of the tides. And so, since humans are so small.. wouldn't the force not weaken as much, and so there is a greater pull towards the moon, making humans susceptible to tides?
    I know i'm wrong, but i dont quite understand..

    Please help! thank you :)
  2. jcsd
  3. Jun 8, 2012 #2

    Jonathan Scott

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    Gold Member

    They could have just as well said "strengthen". The near side of the earth feels the moon's gravity more strongly than average, and the far side more weakly than average. Because the earth is quite large compared with the distance to the moon, the difference is enough to cause significant tides.
  4. Jun 8, 2012 #3


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    Staff: Mentor

    What you are asking about is called a "Gradient". The Moons gravity weakens as you get further from it, and since the Earth is several thousand miles across the field has weakened between the near side and the far side. People are much smaller and while a gradient does exist between the side of our bodies closer to the Moon and the side further away, the difference is much too small to be noticeable.


    It isn't that more massive objects are more susceptible, it is that larger objects have a larger difference in the strength of gravity between the near and far sides. Something being more "massive" refers to the amount of mass an object has, which is kind of like it's weight, not the physical size.

    One thing to understand is that the reason the tides bulge up like they do is that the Moon is pulling the water near it away from the Earth, and it is pulling the Earth away from the water on the far side. Because water is a fluid it easily flows and is able to build up on each side.

    Nope. The difference in strength is very small for a person, while for the Earth it is much larger. When the difference is larger you get larger tidal effects. If I pull the front of your body with X amount of force, and the back of your body with 99.9% of that force, it has much small tidal effects than if I pulled the back of your body with only 75% of the force. In such a case your body would stretch slightly since the front is being pulled more strongly than the back.
    Last edited: Jun 8, 2012
  5. Jun 8, 2012 #4
    Thank you so much for the replies, i now understand! I think i was reading the passage wrong because i somehow didn't think he was talking about the difference on each side but thank you!! :) :)
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