How much weight can the surface of water hold?

by | May 26, 2009 08:00 AM ET

You Asked:

How much weight can the surface of water hold? --- Irvin, Denver, Colo.

Marshall Answered:

If you have ever seen the bug called the water strider skating on the water, you know that the surface of the water he is striding upon can support some amount of weight. If you are patient and careful, you can even place a magnetized needle on the surface of water to make a compass.

Water is able to hold weight like this because of its surface tension. Water molecules are polar, and they are attracted to each other because of that. The attraction gives the surface of the water a certain "strength".

How much weight can water's surface tension hold? Think of the surface of the water as a piece of fabric. That "fabric" has a certain amount of tension it can hold before ripping. So say you took a centimeter wide strip of nylon and you pulled on it. Eventually, by applying enough force, it would rip.

In the case of water, according to this page, the amount of weight is 72.8 dynes/cm (at 20 dgrees C - it gets lower as water gets hotter). That's about 0.000164 pounds/cm, or roughly 0.074 grams. This is for pure water. Tiny amounts of impurities can change that number a lot.

Note that we are Still learning about water striders, and there is more to it than surface tension. See also: Robostrider

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