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Can you stand on a frictionless plane?

  1. Sep 5, 2007 #1
    A friend and I have had an ongoing discussion about whether or not it would be possible for a person to go from a prone position to a standing position on a frictionless surface. He says that it would be impossible while I say that, although difficult, it would be possible after some trial and error. All other variables are exactly like on earth, except the floor would be 100% frictionless. You can't use a wall or other object for aid. It has to be done solely with the human body. So, is it possible?
     
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  3. Sep 5, 2007 #2
    If you can assume the surface to be perfectly frictionless, then you can assume the human body to be perfectly coordinated and balanced too. It would have to aim its force down perfectly perpendicular to the surface, so that no component of it points horizontally, or somehow make sure that all horizontal forces cancel out.
    If it gets too precise, would it then be subject to quantum uncertainty? How perfectly frictionless is the surface?
     
  4. Sep 5, 2007 #3
    I think on the scale of a person you don't have to worry about quantum uncertainty. And what is it meant by a prone position?
     
  5. Sep 5, 2007 #4

    Danger

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    Prone means lying down.
    I suspect that an extremely talented gymnast might be able to pull it off; I know that I couldn't. Then again, I have trouble standing up on ice.
     
  6. Sep 5, 2007 #5
    I think that you could make your body rigid enough to resist the horizontal forces. Look at a chair where the legs aren't perpendicualr to the ground. It's still able to maintain its shape and stand. It would be a similar situation with your arms or legs.

    Prone means lying face down.
     
  7. Sep 5, 2007 #6
    I might say it a little differently.

    In that, where ever your center of mass is when laying down, the forces that make you rise must be all internal (within the body), the center of mass must rise perfectly perpendicularly to the (horizontal) surface from its original location----like a person on ice skates rising up after doing the 'splits'.
     
  8. Sep 5, 2007 #7

    rcgldr

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    One possible sequence. Roll over onto your back. Use stomach muscles to sit up. Place both hands on right side, and bend both knees with lower legs to left side, with right knee close to left foot. Both hands, right knee and right ankle form a rectangle on the surface at this point, with the right lower leg on surface. Press down with right ankle to rotate and raise hips to end up on both hands, left knee, and side of right lower leg. Adjust left knee for balance with both hands by sliding it into place. Raise right knee and place next to left knee. You're know in a crawl position. Raise one leg and place foot (or ball of foot, depends on flexibility here) on ground. Balancing with two hands and the ball of one foot, raise other foot. Shift weight back so it's on both feet (or balls and toes of feet), and stand up, using arms for balance.
     
  9. Sep 5, 2007 #8

    DaveC426913

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    Well frankly - the centre of mass will rise prependicularly, whether the person wants it to or not. The frictionless surface will ensure that.

    The only things that need to happen are
    1] the centre of mass must stay inside the supports or he'll fall over
    2] the person will probably have to have excellent muscle control - like your skater rising from the splits
     
  10. Sep 5, 2007 #9
    UtterMess, I agree with you: difficult, but possible.

    Ask your friend what's wrong with this: if you have sufficient strength, you will be stable if you support yourself at four points on the ground in such a way that your center of mass is directly over some point inside these four points (or inside the quadrilateral they define, to be a bit more precise) - think like a table with splayed legs. So, what you do first is to spread your arms and legs out "spread eagle" style, and then slowly bring them towards each other as you lift your body upwards (I said you'd need sufficient strength!). Eventually you'll be standing on your feet and hands, bent over, right? Now, if you slowly lean backwards until your center of mass is over a point contained within the quadrilateral defined by your toes and heels (i.e. another four points), and if you have a good enough sense of balance, you should be able to straighten up in a stable fashion, keeping that C-O-M over your feet. They might slide forward or backward as your C-O-M moves backward or forward, but who cares? The point is that you'll be stable.
     
  11. Sep 5, 2007 #10

    LURCH

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    Yeah, what Belliott said. You have four points (heel and toes on each foot), so friction is not necessery. But for bonus points, could the person stand still (in one spot)?
     
  12. Sep 6, 2007 #11

    yep --I was just thinking without using any 'extra' energy, I guess
     
  13. Sep 6, 2007 #12
    air drag???
     
  14. Sep 6, 2007 #13
    That's what I was thinking ank, air resistance would be a great help to get you onto your feet.
     
  15. Sep 6, 2007 #14

    You wouldn't even need to go that far. You could just do a pushup with your legs spread. Much easier and gets you into a similar ending position.


    Not sure how much of an effect it would have either way. It would slow you down if you started sliding, but thats about it.
     
    Last edited: Sep 6, 2007
  16. Sep 8, 2007 #15

    cks

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    Conditions:

    1)assuming, that person is standing inside a train, with all the windows closed, (so no air drag)
    2)only floor and the human body contact

    2 cases
    1st case (train is accelerating)
    the person cannot stand still on the floor, because there is no force acting on the person to make it accelerate with the train as well.

    2nd case(train is travelling at a constant speed)
    the person can stand still on the floor.
     
  17. Sep 8, 2007 #16

    cks

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    the 2nd case is similar to what the UtterMass said, it's possible. consider the earth as moving at a constant velocity in an absolute frame(which doesn't exist.)
     
  18. Sep 8, 2007 #17

    LURCH

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    I agree. At first I thought, "the person could stand, but how could they keep one place without friction to hold them?" Then I realized that, without friction to propell them, the person actually couldn't move from their place! If we allow normal aerodynamic effects, then they could use the friction of the air to "swim" across the room, but if we had frictionless floors and frictionless air, then the person had better be within reach of a wall at the beginning. Even with frictionless air, floors and walls, a person would be able to move about, so long as they never found themselves at a dead stop relative to the room, with no walls within reach. In that state, the person would be trapped for ever.
     
  19. Sep 8, 2007 #18
    Trapped yes, but still able to stand.
     
  20. Sep 8, 2007 #19

    DaveC426913

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    I'm not sure even frictionless air can ignore Newton's 3nd law. She may not be able to swim, but he can still generate propulsion. Turn head left, inhale; turn head right, exhale. Repeat.
     
    Last edited: Sep 8, 2007
  21. Sep 9, 2007 #20

    Danger

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    I didn't know that we were allowed to quote the 'Kama Sutra' here.
     
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