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Questions about gravity

  1. Jul 10, 2006 #1
    I have two questions right now, so if someone could answer them, I'd be greatful.

    When your center of gravity isn't supported very well, for example, when you lean over too far, it is difficult to stand, and not fall down, could someone explain why this is?

    Second, if gravity was stronger(though I'm not sure how strong it would have to be), would it pull us through the ground? I'm just wondering, because I know that the normal force keeps us from accelerating into the ground, but is there a limit to the upwards force the ground can apply to us?

    That's all for now, thanks in advance for any help I get.
  2. jcsd
  3. Jul 10, 2006 #2
    I think I understand the context of the questioner. If you lean over, it is impossible to stand! What prevents you from falling is your muscles. There is only one way to really stand, and that is perfectly straight. Anything else is just your muscles preventing the fall.

    Or please read the next 25 posts for a more detailed explanation...
  4. Jul 10, 2006 #3


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    actionintegral is right on the first question. When you stand directly upright, the weight of your body is "transmitted" directly down through your skeleton, requiring no muscular effort at all.

    When you are not balanced, the only thing keeping you from toppling is the tension in your muscles, opposing gravity.

    As for the second question: if gravity were stronger than the electromagnetic forces that prevent us from breaking the floor, then yes, we'd fall through the floor. The sensation of "touch" is really just the electromagnetic repulsion of atoms in one surface by atoms of the other.

    - Warren
  5. Jul 11, 2006 #4


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    Assuming that your feet or shoes aren't glued, nailed, or otherwise attached to the floor, you're lean is limited to a having a point of support under your center of gravity. Since your feet / shoes point forwards, you can lean forwards as long as your center of gravity doesn't go beyond the front edge of the contact point of your feet or shoes with the floor.

    If you feet / shoes are attached to the floor, then it's a question of how much torque your muscles can generate to counteract the torque if your center of gravity is outside the outermost contact point with the floor.
  6. Jul 11, 2006 #5
    I noticed two common conditions in this respect. Standing errect/straight(I can't same about bending backward) and slight bending forward. One may fell weak or strong spine related to these.

    Is it related to changes in vascular's muscles tones--relaxed/contracted condition or measure of "local blood flow control--acute or chronic" mechanism.
  7. Jul 11, 2006 #6
    It's because when you're well supported your bones are taking the weight, and it's no real effort to stand up straight. But if you lean over your muscles are working, and it's tiring.

    No. If gravity were stronger, the things that make up the ground would be pulled downwards too, and the ground and would be compacted. So it would be stronger, and we wouldn't be pulled into it.
    Last edited: Jul 11, 2006
  8. Jul 25, 2006 #7
    Any time the line of gravity which goes through your body's center of gravity extends beyond your base of support, you fall over...ie when your center of gravity (normally located in the rear of the pelvic area, sacrum 2, to be exact) extends beyond your base of support (the part of you that's touching the ground or a seat or whatever), you fall over because of gravitational pull.

    Think of a plate on the edge of a table. The moment the plate's center of gravity is beyond the table's edge (its base of support), it falls to the floor. This is true for the human body as well.

    Hope this answers your question. I just learned about this in a physical therapy class.
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