2010-11-12 Archive, Words, Falling Water

Falling Water.

I was rinsing the residue out of a can under the slow stream of water from the kitchen faucet when I noticed something: When I hold the can close to the faucet, the pressure of water hitting the can is light, but when I hold the can down away from the faucet, then the pressure of water hitting the can is greater.

Obviously, as the water comes out of the faucet it starts to fall. Now, everything that falls, speeds up under the force of gravity. So, the lower I move the can under the faucet, the faster the water is when it hits the can.

Then I started to think about waterfalls. Why is it that the water at the top of a waterfall can be smooth and continuous, but as it falls it is broken up into thousands or millions of drops?

I will readily admit that I have not thought about this very deeply. Before today I would have answered this question by saying something like, "The resistance of the air against the water falling through it probably breaks up the water into small drops." And I would have been wrong.

I already know that water is kind of sticky. It has a certain resistance to letting go when it is touching something. Also, water has something called surface tension that helps hold it together, and causes it to bead up into drops when the water is on a flat surface. So, I did an experiment.

I turned down the flow of water coming through the faucet until it was a smooth stream about an eighth of an inch (three millimeters) wide. For about three inches (seven centimeters), the stream of water remained smooth, but appeared to narrow in width until it reached the end of its smooth flow, where it broke up into drops.

So, what was happening?

1. The water came out of the faucet at a rate which was fast enough for the surface tension to hold the water in the form of a narrow pipe.

2. As the water fell, it accelerated, stretching the stream of water into a narrower form of pipe. I knew from physics class that water moving through a pipe speeds up when the pipe narrows. But who knew that speeding up the flow of water stretches it into the form of a narrower pipe?

3. At some point, the water overcomes the surface tension holding it together. Then the water is pulled apart into separate falling drops. Note, this would still happen without any wind resistance at all.

4. How could we test or eliminate the possible effect of wind resistance to the falling stream of water? Just put the narrow stream of water into a larger clear tube, where the air around the water is moving down the tube at the same speed as the falling water. Then the water should still break up into drops due to its own speed in falling, not due to air resistance.

So, the next time you watch a waterfall as it moves from a smooth stream of water into a cascading veil of shredded water drops, you'll know why it does what it does.

Think well,
Logan