Hourglass floating through a tube of water

[pier4r]

http://i.imgur.com/Ij1nH7t.gifv

[BobVA]

It's cheating to watch it for more than one trip up the tube, because then you can figure out what's happening :-)

Bob

[SlideRule]

to Martin Gardner & his many puzzler publications...
"One hourglass is slightly positively buoyant, and the other one is slightly negatively buoyant. So the starting position is that one glass is at the top of its tube, and the other is at the bottom. However when you turn the device upside down, each inverted hourglass now has sand at the top, and air at the bottom. This makes it top heavy, or bottom buoyant if you like, and it has a tendency to try and flip over. However it cannot do this because it fits fairly snugly within the tube. But the effect is that it wedges itself in, and it is held in place by friction. Technically this is static friction, which is sometimes called 'stiction'. As the sand falls through the hourglass, its tendency to flip over is reduced, until it 'unsticks' from the side, and positively buoyant glass floats to the top, and the other descends to the bottom. The trick depends on the two hourglasses being only slightly positively or negatively buoyant. Were this not the case, their natural buoyancy would be strong enough to overcome the 'stiction' effect immediately, and the trick would not work".

BEST!
SlideRule

[Tugdual]

Lol I think this is more like a magic trick than actual physics.
With no sand the move would be exactly the same; it might be glycerin instead of water, slowing down the movement. The sand is just there to blow your mind.
Funny....

[pier4r]

actually slide rule should have it right.

[Joe Horn]

(09-07-2017 10:05 AM)Tugdual Wrote:  Lol I think this is more like a magic trick than actual physics.
With no sand the move would be exactly the same; it might be glycerin instead of water, slowing down the movement. The sand is just there to blow your mind.

If that were true, then the hourglass would begin to ascend as soon as the tube is inverted. But that's not the case; the hourglass stays motionless at the bottom for a while (some of that time was unfortunately edited out of the video, but it can be deduced by watching the sand level at the bottom), and only then it begins to ascend. So something must have changed to start it ascending... but the only thing that changed was the sand (I think). So the amount of sand in the top and bottom must have something to do with whether the hourglass is either stuck at the bottom or ascending. SlideRule's quote sounds like a reasonable explanation of that to me.

[Tugdual]

(09-07-2017 12:13 PM)Joe Horn Wrote:  

(09-07-2017 10:05 AM)Tugdual Wrote:  Lol I think this is more like a magic trick than actual physics.
With no sand the move would be exactly the same; it might be glycerin instead of water, slowing down the movement. The sand is just there to blow your mind.

If that were true, then the hourglass would begin to ascend as soon as the tube is inverted. But that's not the case; the hourglass stays motionless at the bottom for a while (some of that time was unfortunately edited out of the video, but it can be deduced by watching the sand level at the bottom), and only then it begins to ascend. So something must have changed to start it ascending... but the only thing that changed was the sand (I think). So the amount of sand in the top and bottom must have something to do with whether the hourglass is either stuck at the bottom or ascending. SlideRule's quote sounds like a reasonable explanation of that to me.

Succion effect. At first there is no liquid below the hourglass hence all pressure is on top. When the hourglass progressively moves it reaches nominal vertical speed.