**Today’s Topic: Bernoulli Principle.**

Before I start up with Bernoulli, what is the Venturi principle? The idea is pretty simple and more intuitive than the Bernoulli’s.

Take a fluid (air qualifies under certain conditions), moving at a certain speed and force it through a narrower space. The speed of the fluid will increase. This principle is used in carburetors.

Daniel Bernoulli took it one step further in his paper “Hydrodynamica” published in 1738. He showed that the pressure exerted by the faster moving fluid *decreases*. It will increase again when the fluid slows.

The other thing you need to know is that a pressure force will always point toward lower pressure whenever a difference in pressure appears.

There are numerous demonstrations of the Bernoulli principle. One oft used is a beach ball suspended in mid-air over a stream of air. The air stream hitting the bottom of the ball is moving faster in the middle than on the sides therefore lower pressure exists right against the ball. This inward-directed pressure force keeps the ball in the middle of the airstream.

Other examples include a table tennis ball in a funnel, round cheese balls (like Cheetos only round) and soda straws, air blown between two empty two liter bottles and air blown over paper. Toilet paper rolls and a leaf blower make a pretty dynamic demonstration too!

A wonderful interactive example of the Bernoulli principle is located at this web site.

What I found most surprising during my investigation of the Bernoulli principle is the debate about how to best describe how an airfoil works or: what really gives a plane wing lift? The other surprising item is that some text books are stating that air which separates above and below a wing must meet up exactly on the other side, forcing it to go faster on top of the wing and setting up the pressure difference required by Bernoulli’s principle. Experiments have clearly shown that the air parcels never meet again after the wing splits them up.

Next Week’s Topic:

**Fun Science Principles With 2 Liter Soda Bottles**