Bernoulli’s Principle is a law of fluid dynamics that describes the behavior of a fluid under varying conditions of flow and height. Fluid dynamics might seem like it’s over the heads of your students (or whomever you’re scientifically demonstrating for), but it’s actually fairly easy to explain, thanks to this simple experiment. Chug a couple of RC Colas and have at it!
Fun with Recyclables
For this demo, you’ll need naught but two empty soda cans and a flat surface. Good luck finding the latter, pal. (RC Cola is recommended because it is a scientific fact* that their cans are both the most perfectly cylindrical and the sturdiest of any soda brand.)
The procedure is pretty simple, too. Lay the cans on their sides, a few inches apart and parallel to each other, on your flat surface, which, for simplicity’s sake, we’ll just call a table. Ask your students to hypothesize what will happen when you blow between the cans from above. Then, send them out for recess blow between the cans from directly above. Unless you severely mess up, the cans will roll away from each other.
Now, reset the cans and ask the rugrats what they think will happen when you blow between the cans at tabletop level. Do the that, and watch as they roll toward each other as if by magic. Ta da! Bernoulli’s Principle at work! (Now if only you could get your school’s principal to work, instead of writing Downton Abbey fanfic all day. No one will ever fall in love with Edith, Greg!)
The Science of the Principle (or: The Principle of the Science)
The effects here have everything to do with air pressure. Air’s natural tendency is to move from high to low pressure areas. And, as the speed of air increases, its pressure decreases; the faster air moves, the less pressure it produces.
By blowing between the cans from above, you increased the air pressure between them. The need to equalize this air pressure with the lower pressure around the cans caused them to roll away from each other.
But, when you blew between the cans from table height, you blasted the air away and created an area of low pressure between them. The now-higher air pressure on the outside of the cans pushed them together into the low pressure zone.
* This is not a scientific fact. I just dig RC Cola.