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Q & A: Concave handkerchief

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Most recent answer: 10/22/2007
Q:
I did an experiment, and the book and my mommy and daddy couldn’t explain it. We put a wet handkerchief over a full glass of water. We fixed the handkerchief with a rubber band. When we turned the glass upside down, the water molecules formed a seal so that water stayed in the glass, but the handkerchief was concaved towards the bottom of the glass. Why was the handkerchief concaved?
- Giselle Serate (age 7)
Rancho Palos Verdes, California
A:
Water hardly compresses or expands when it is a liquid and the pressure changes. The volume of the water doesn't change when you turn the glass upside down. If there was a little bit less water than to fill up the glass so that the wet handkerchief was in a concave position when the glass was upright, then the handkerchief must be in a concave position when the glass is inverted in order to keep the volume of the water constant.

Of course wet handkerchiefs are notoriously leaky, and water was probably dripping out all the time. As water drips out, the volume of what's left is even less, and the handkerchief should get even more concave over time. If air leaks in, the handkerchief could become convex. If your water glass were half full at the beginning, it is concievable that the air could expand under the lower pressure and allow the handkerchief to be convex, but the dripping out of the water would make it concave again soon.

Tom

(published on 10/22/2007)

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