Bernoulli Eggs

This week's experiment comes from an observation by a subscriber. They noticed that as they were filling the bathtub, a toy ball in the tub was attracted to the stream of water and stayed in the stream, instead of being pushed away. This time, we will take a look at the forces involved in holding the ball in the stream of water.

The last time that I did an experiment using the bathtub, my Mom jokingly accused me of encouraging people to take a bath with a friend. To avoid that notion, this time we will move the experiment to the kitchen sink. You will need:

  • a shallow pan or pot
  • an egg, plastic ball or other round object
  • the faucet

Place the egg into the pan and put the pan in the sink. To keep from wasting water, you might do this while you are filling the sink to wash the dishes. (You do help with the dishes, don't you?) Turn on the water and hold the pan so that the egg is against the side and the water is hitting between the egg and the side of the pan. Now tilt the pan so that the side away from the egg is lower. The egg will remain against the higher side of the pan, even though you would expect gravity to pull it downwards. You will find that you can tilt the pan quite a bit before gravity finally pulls the egg out of the stream.

What you are observing is known as Bernoulli's Principle. Daniel Bernoulli was lived from 1700 to 1782. He made many important discoveries in science and mathematics. One of these discoveries had to do with moving fluids. He found that as a fluid moves faster, the pressure that it exerts towards the sides was less. The faster the fluid moved, the less sideways pressure there is.

How does this impact our egg? The moving stream of water is pushing less on its side of the egg. The still or slower water on the other side is pushing more, so the egg is continually pushed into the stream of water. At first this may be a little hard to understand, but that will give you something to think about as you wash the dishes.