Bernoulli's Theorem, also known as Bernoulli's Principle, states that an increase in the speed of moving air or a flowing fluid is accompanied by a decrease in the air or fluid's pressure. This theorem can be explained to kids via a simple experiment with a plastic bottle and a ping pong ball. Follow these steps to explain Bernoulli's Theorem to kids.
Prepare the plastic soda or water bottle for the experiment. Use your scissors to cut off the top portion of the plastic bottle. You will want to use the spout or mouthpiece of the bottle plus about two inches of the bottle. Discard the bottom portion of the bottle.
Place the ping pong ball in the plastic bottle and blow upward through the bottle's mouthpiece. You will note that you cannot blow the ball out of the plastic bottle due to Bernoulli's Theorem. In fact, you will note that the harder you blow on the plastic ball, the tighter the ball stays in the plastic bottle.
Talk to the kids about airflow around a curved surface such as a ping pong ball. When a ball or other curved object is placed in an air stream (such as in Step 2), the air will increase its speed as it moves around the outside of the ball. This happens because the air has to travel a further distance to get around the ball and meet back up on the other side of the ball.
Mention the connection between air speed and air pressure that is at the center of Bernoulli's Theorem. When the air increases its speed as it moves around the ball, the air pressure around the ball also drops. In the places where the air moves the fastest, the air pressure is also the lowest.
Explain that the low air pressure around the ball pulls the ball into the plastic bottle. When you or a student blows hard on the ball, you increase the speed of the air around the ball. This also causes the air pressure to decrease, which then pulls the ball even further down into the plastic bottle.