Day 10 - Bernoulli's Principle

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Day 10: Bernoulli's Principle

Teacher information:

This lesson focuses on Bernoulli's Principle. This principle states that for a fluid undergoing steady flow, the pressure is lower where the fluid is flowing faster. In this lesson, the "fluid" with which we are concerned is air. Fluids include liquids and gases and air is a mixture of gases and therefore a liquid.

The first activity has students making predictions to confront their own ideas about airflow and its effects. Most students, when asked what will happen when air is blown between two suspended ping pong balls, will believe that the balls will fly apart from one another. Instead, the balls are drawn toward one another. Students then carry out other activities with the Bernoulli principle and attempt to explain what happened in the first activity.

Air that is not moving pushes in all directions with equal force. Thus air is pushing down on the top of a table top and up on the underside of the table. Moving air however pushes to the side with less pressure; if a fan pushes air across the top of a table the downward pressure on the table is reduced.

When a plastic ball is resting on your hand, air is pushing in on all sides of the ball with the same force. As you start moving your hand (and the ball) into an air stream, as students will in the second activity, there is reduced pressure on that side of the ball. Move the ball far enough into the air stream and there is such a difference in pressure that the still air on the back side of the ball pushes it off your hand and into the air stream. The same thing is true of the ping pong ball in the funnel. The side of the ball that is not being blown on has more air pressure so the ball moves that direction.

Here are some other examples:

In a storm where the wind is blowing very hard, the pressure of the air inside a house is higher than the pressure of the air blowing across it. This can lift the roof off of a house.

If you are carrying a large sheet of plywood in the bed of a pickup truck, you may very well want to watch it carefully, since the air moving past the truck is at a lower pressure than the air trapped under the plywood, and it could be raised up high enough to catch in the wind and be blown away.