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A cup that moves by itself.

  1. Aug 17, 2009 #1
    Please bear with me because I'm not that great at physics. I witnessed something earlier today which was the most bizarre thing I have ever seen, and I'm looking for an explanation. I know this sounds contrived but this is exactly what happened without any embellishment: I was at work making two cups of coffee and I poured water from the kettle into the first cup and then as I was pouring water into the second cup, the first cup moved, entirely of its own accord and without me touching it. It turned through 180° whilst sliding to about 2 inches from where it started and was still moving when I grabbed it.

    So what I think happened is: the kettle is not very good and when you pour the water always dribbles down the spout and pools on the table. So I think that the cup must have been planing on the surface of that water. I reckon the motion of the water inside the cup sloshing around after being poured must have somehow translated into moving the cup. Is this possible? It seems unlikely for something with the mass of a cup of coffee, but I can't think of any other explanation.

    In case it is relevant: the surface it was on was a laminate kitchen worktop.
     
  2. jcsd
  3. Aug 17, 2009 #2

    Danger

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    Welcome to PF, Nick.
    I'm not sure if the liquid motion would account for it, but I suppose that it's possible. I have, however, experienced the exact same phenomenon when there was a vibration present in the (dry) surface. The source of that could be a microwave oven turntable motor out of balance, or even just the kettle itself trembling if it was still boiling. If you work in an industrial environment, it could even just be something transfered through the floor from a milling machine or suchlike, or from construction going on anywhere in your neighbourhood. The last time that it happened to me, they were digging up a sewer line almost a block away.
    In those cases, I also saw concentric rings like tiny waves forming on the surface of the liquid. Do you notice anything like that?
     
  4. Aug 17, 2009 #3

    Mapes

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    And I've also experienced the same thing myself. I concluded (at the time) that the hot liquid was heating the air under the mug (there was a slight recess, a small dome-shaped region, under the mug). As the hot air vented, the mug behaved like a little hovercraft, sliding and rotating on a smooth countertop with essentially no friction.
     
  5. Aug 17, 2009 #4
    Thanks for the replies... I don't think the movement was caused by vibrations as there were none noticeable and it was a very smooth, gliding movement and I would imagine that vibrations would cause juddery motion. Mapes seems to be describing the same thing I saw, but if the cup is experiencing an essentially friction-free surface, would you not still have to apply a force for it to move? If that is the case and considering that only a very small force would be required then would the liquid motion be sufficient? If not, perhaps an uneven venting of the hot air?
     
  6. Aug 17, 2009 #5

    Mapes

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    The force could come from venting, as you said, or the gravity component parallel to the countertop (which is never perfectly flat), the fluid momentum when you poured the liquid, air currents, etc. Don't know which was dominant in your case.
     
  7. Aug 17, 2009 #6

    Borg

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    The spilled water is probably the reason. My mechanics professor considered putting this exact problem on a final exam. This was 25 years ago so I'm quite rusty on the details. I do remember that the suface tension of the water under the glass was something that we had to consider. With the right conditions, the cup actually floats on the water. Then, all you need is the slightest of forces to move it - could be a slosh or even a slight variation in the level of the counter.
     
  8. Aug 17, 2009 #7

    Danger

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    I agree that the countertop was probably not level, and that the planing effect is quite plausible. One point of curiosity, Nick. You said that the cup rotated 180°. By any chance, was the handle facing away from the direction of travel when it started, and ended up facing toward it? That would be evidence for it travelling down a slope, since the handle side would be heavier.
     
  9. Aug 17, 2009 #8

    turbo

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    Alternately, if expanding air needs to vent from under the cup, it would tend to vent out from under the lightest side of the cup. No slope needed if the counter-top and the bottom of the cup are smooth.
     
  10. Aug 18, 2009 #9

    Danger

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    Good point, Turbo.
     
  11. Aug 18, 2009 #10

    berkeman

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    Hah! That's downright brilliant, Turbo. :biggrin:
     
  12. Aug 18, 2009 #11
    Yes, that's right. The handle started facing away from the direction of travel and finished facing the same way.
     
  13. Aug 18, 2009 #12

    DaveC426913

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    I thought you said it turned through 180 degrees?

    Cuz I was going to theorize that it pivoted on a tiny bump on the bottom rim. Would explain both the rotation and the 2 inches of travel.
     
  14. Aug 18, 2009 #13
    Err... that is 180 degrees?
     
  15. Aug 18, 2009 #14

    DaveC426913

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    If the handle started off facing away from the direction of travel and finished the same way, I interpret that as a net rotation of zero degrees.

    To have rotated 180 degrees, the handle would have to end up facing toward the direction of travel.
     
  16. Aug 18, 2009 #15
    Oh, I see... what I meant by 'the same way' was 'the same way as the direction of travel', not 'the same way as it started'. Sorry, I didn't make that very clear.
     
  17. Aug 18, 2009 #16
    My wife & I saw this several years ago in a Chinese restaurant - the small stainless steel teapot started moving across the glass-topped table. We still laugh about it whenever we get tea in a pot like that... I think the spout dripped a little hot tea and the table had a slight slope to it.
     
  18. Aug 18, 2009 #17
    @Nick
    So there was no rotation. only translation. right?
     
  19. Aug 18, 2009 #18

    DaveC426913

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    Wrong. It rotated through 180 degrees. Handle started off pointing away from direction of travel and ended up pointing toward direction of travel.
     
  20. Aug 18, 2009 #19
    oh i now see it. But i have a doubt. How can the water poured down on the cup explain the apparently gliding motion? Since the water molecules at the time of collision with the cup should be having only a vertical component of velocity, the cup and water as a system cannot have a net horizontal velocity. I think that brings us to the theory of reduced friction brought out by the heated air; just like dry ice sublimation reducing the friction in this video helping the flea to carry the heavy load.
     
    Last edited by a moderator: Sep 25, 2014
  21. Aug 19, 2009 #20

    DaveC426913

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    I don't think anyone is suggesting that.

    The movement of the cup is almost arbitrary, the idea here is that the water and/or heated air is reducing the friction to zero. Once friction is zero, the cup can move easily with virtually no provocation whatever.
     
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