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Freely falling elevator

  1. Aug 16, 2012 #1
    There is a drop of mercury in an elevator (flat on the ground).
    during freefall what will happen to the mercury.
     
  2. jcsd
  3. Aug 16, 2012 #2

    mfb

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    It should form a (nearly) perfect sphere, probably floating upwards very slowly.
     
  4. Aug 16, 2012 #3
    what causes it to float upwards
    plus if it floats upwards then it should oscillate inside the elevator after touching the top.
     
  5. Aug 16, 2012 #4

    jbriggs444

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    I think the idea is that as the elevator starts to fall, the formerly-flat blob of mercury will fatten out to form a spherical shape. In the process of doing this it will effectively "push off" from the floor.
     
  6. Aug 16, 2012 #5

    mfb

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    Initially, the drop is flattened a bit by gravity (or by the floor pushing it, if you like). If you "switch gravity off", the mercury gets a small kick from this force, until it loses contact with the floor. After some initial, damped oscillation, it reaches a stable spherical shape.

    I would not expect that it reaches the top before the elevator crashes, but this might require some simulation (or experiment ;)) to find out.
     
  7. Aug 16, 2012 #6
    I don't understand what is mean by switching off gravity gives the mercury a kick.
    If I simply drop a plate with water on it I don't think the water gets a kick....it just falls like the plate.
     
  8. Aug 16, 2012 #7

    jbriggs444

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    The assumption is that the mercury is not wetting the floor of the elevator. It is beading up. If you want to compare with a plate of water, you will need to grease the plate or otherwise arrange for it not to be wetted. That way the water will "bead up".

    Now before you drop the plate, a water bead will be flattened somewhat into a pancake shape, right?

    And after you drop the plate that bead will draw itself together into a sphere, right?

    Where is the center of gravity of the pancake shape with respect to the plate?

    Where is the center of gravity of the spherical shape with respect to the plate?

    Does this imply that the water bead has moved away from the plate?

    Does this imply that the water has acquired a velocity relative to the plate?

    Where did that velocity come from?
     
  9. Aug 17, 2012 #8
    So this explanation must be due to surface tension?
    If I drop a plate with ball bearings on it the ball bearings don't get a kick, do they?
    Where is the centre of mass of the plate with respect to the drop........
     
  10. Aug 17, 2012 #9

    mfb

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    They will compress the floor (and in theory even themself) a bit, the corresponding restoring force would give them a small kick, too. But the effect would be smaller, and negligible if the floor is some hard material.
     
  11. Aug 17, 2012 #10
    I'm guessing this kick is the point were the mercury separates? Also off on this no-gravity kick thing.
     
  12. Aug 17, 2012 #11

    mfb

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    The kick is done before separation. It does not come from gravity, but the situation before the elevator falls is necessary to get it. Think of it like a spring which gets compressed by gravity, when the elevator is resting. In free fall, the compressing force is gone, and the spring extends.
     
  13. Aug 17, 2012 #12
    Easier to visualize than to describe for me, but I understand what you're saying.
     
  14. Aug 17, 2012 #13
    now its all clear... http://kontaktniy.org [Broken]
     
    Last edited by a moderator: May 6, 2017
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