Floating Hourglass - Answer


Way back in 1992, one of us (Tim Rowett - the guy in our videos) sent one of these to Scot Morris in the USA, who wrote an article about it for OMNI Magazine. He explained what happened, and invited readers to come up with an explanation. Four months later, he had received 415 correct answers, and 618 incorrect answers.

Some people thought that heat was a factor - they thought that the falling sand generates heat, which heats the surrounding liquid, and so the hourglass stays down until the water cools down again. Although some people thought the falling sand heated up the air inside the hourglass, making it expand slightly, and thus rise.

Other solutions involved a flexible hourglass - when the sand is falling, it presses down, and causes the hourglass to widen, thus wedging it in position, until the sand stops falling. Somethought it was to do with the 'impact' of the falling sand hitting the bottom of the glass.

There were a variety of other explanations, including some that thought the whole thing was an illusion! However the correct answer is as follows.

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.

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