force and motion | thehappyscientist.com

Last week, I left you with a question about the behavior of a stream of flowing water. I love things like this, because they are things that we see every day, and never really stop to wonder about. Once you do start to look and think, you see wonders that you never noticed before.

We were looking at a stream of water from the faucet, and noticing that as the stream fell, it got thinner and thinner. Now, why would it get thinner?

To understand that, we need to think a bit about how things fall. If you hold a ball out in front of you and drop it, it falls downwards. In the short drop to the floor, you may not notice anything else, but if the drop is longer, you will notice that as the ball falls, it is speeding up, falling faster and faster. That is because gravity continues to pull on it, so it accelerates. The acceleration of gravity is 9.8 meters per second per second. No, that second "per second" is not a typo.

What that tells us is that if you drop the ball, after one second, be falling at 9.8 meters (32 feet) per second. During the next second, it would have been accelerated by the pull of gravity to19.6 meters (9.8 + 9.8) per second. In the third second of its fall, it will be falling at 29.4 meters (9.8 + 9.8 + 9.8) per second.

The process continues, with the ball falling faster and faster, until the resistance of the air balances the acceleration of gravity. At that point, the object won't fall any faster. That is called the terminal velocity. Terminal velocity keeps raindrops from punching holes in the roof, and keeps pennies dropped from sky scrapers from making holes in the sidewalk.

So what does all that have to do with our water? Well, as the water falls, it is speeding up too. That means that the bottom part of the stream is falling faster than the top part. If we were looking at a stream of sand grains, maybe the sand falling in an hourglass, we would see that the grains of sand were very close together as they began their fall, and that they spread farther apart as they fell.

Then why doesn't the water do that? Well, water molecules are very sticky. They stick to each other very strongly, which is what causes surface tension. That keeps them from spreading out, so instead, the stream is stretched and pulled into a narrower stream, sort of like pulling on a piece of chewing gum.

As the stream of water falls faster and faster, the stream is stretched thinner and thinner. Eventually, the stretching reaches the point where it overcomes the stickiness of water, causing the stream to break up into separate water drops. You can see that by barely turning on the faucet, to create a very thin stream of water. Near the bottom of the stream, you will see it starting to break up into drops.

Sometimes it helps if you slow things down a bit, to make them easier to see. Galileo did that with the acceleration of gravity, studying balls rolling down in inclined plane instead of falling. We can do the same thing, with something thick, like a stream of chocolate syrup, falling onto a bowl of ice cream. Does it behave in the same way as a falling stream of water? You will have to try it yourself to find out. Now that is some fun homework!

Have a wonder-filled week.