1
$\begingroup$

In http://what-if.xkcd.com/6/,

Around this time, the glass on the left starts to visibly lift into the air.

The air pressure is trying to squeeze the glass and water together. This is the force we think of as suction. The vacuum on the right didn’t last long enough for the suction to lift the glass, but since air can’t get into the vacuum on the left, the glass and the water begin to slide toward each other.

The glass on the left has a bottom half of vacuum, and a top half of water. I do understand that the air is pushing on the water from the top, causing "suction". I don't dispute the water filling the vacuum.

Now, I would think that, with a free-floating litterally half-empty glass of water, the glass and the water would be pushed towards the vacuum, and the glass would "rise", because it is pushed by the air mostly from the bottom, and less from the top rim of the glass. But the illustrations show the glass is on the table. There is no air to exert upwards force on the glass.

The only way the glass could rise is by the angled sides, which would provide some upwards force, though I am not sure they are part of the problem. In addition, the glass is show to have enough momemtum to hit the ceiling.

Is this possible with or without the angled sides?

$\endgroup$
2
$\begingroup$

... the glass would "rise", because it is pushed by the air mostly from the bottom, and less from the top rim of the glass. But the illustrations show the glass is on the table. There is no air to exert upwards force on the glass.

Drinking glasses don't tend to form pressure boundaries with the table they're sitting on. If they did, we'd call it a suction cup and it would be difficult to lift.

So you can assume that although it is sitting on the table, there is still air pressure present on the bottom of the glass. Is the volume sufficient to keep the pressure high as the glass rises, I'm less certain.

$\endgroup$
0
$\begingroup$

Even if the glass is sitting in contact with the table, as illustrated, some air can still get in between the table and the glass. There isn't an airtight seal. To achieve that you would need something like a suction cup on the bottom of the glass.

$\endgroup$
  • $\begingroup$ Arg - you beat me to it. I'm going have to practice typing faster... $\endgroup$ – M. Enns Mar 31 '16 at 17:54

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.