Feb 10, 2011

bottomless cups of sand

Take two cans.

  1. Cut the top off of one but leave the bottom on.
  2. Cut the top and bottom of the other one (it becomes a tube)
Turn both cans upside down and conduct two experiments.

Experiment A
Hold the cans over water and push down into water.  Which can pushes back with more force?  Clearly the one with a bottom as the air becomes trapped inside the can making it more buoyant than the one without a bottom.

Experiment B
Hold the cans over sand and push down into the sand.  Which can pushes back with more force?  Now I'm sure you can guess that something unusual happens here so I'm sure you'll say the can without a bottom pushes back with more force.  But the question is why?

Here's a video (doesn't seem to be embeddable)

So what's going on here?  As explained by the authors of a new paper (the guys in the video) the can with a bottom forces any air out at the edges of the can as it is pressed down.  This air passing through the sand causes the sand to be more like a liquid and less like a solid.  So effectively the can is passing through a liquid-like substance.  Compared this to the can with holes in the bottom.  The air in this case passes out through the holes.  The junction between the sand and can in this case has no air passing through and so the sand behaves more solid-like.

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