Science in Sweatpants: Exploring the Bernoulli Principle

The Bernoulli Principle states that "as the velocity of a fluid increases, its pressure decreases." This was discovered by Daniel Bernoulli, a Swiss physicist and mathematician, in the 18th century.

Today we’ll demonstrate the Bernoulli Principle in two different ways. Adult supervision is recommended for both.

Here’s what you’ll need for the first demonstration:

  • Two balloons
  • Two pieces of string, equal in length
  • A straw
  • Push pins or tape

Inflate your balloons and tie one piece of string to the end of each one. Using your tape or push pins, secure the other end of the string to the top of a door frame or some area where the balloons can hang freely. The balloons should be 2"-4" apart, not touching.

Using your straw, blow air between the two balloons. Before you do this, hypothesize what you think will happen. Will the balloons drift away from each other or come together?

Make sure you blow hard enough to create a stream of air between the balloons but not so hard that you blow them away.

When you blow between the balloons, they should come together and touch. According to Bernoulli’s principle, you’ve increased the velocity of the fluid, the air, between the balloons and so the pressure between the balloons has decreased. The balloons are drawn to the area of lower pressure, and eventually come together.

For the second demonstration, you’ll need:

  • A hair dryer
  • One or two ping pong balls

With your hair dryer pointed straight up, turn it to high and drop the ping pong ball or balls in the air stream.

The ping pong ball is held up in the air current, even when you tilt the dryer slightly.

The air produced by the hair dryer has a higher velocity than the air surrounding it so, according to the Bernoulli Principle, the pressure in the air current is lower than the surrounding air.

The oscillation or wobble is caused by the ping pong ball trying to escape the air stream, an area of lower pressure, and being pushed back in by the higher pressure surrounding the air stream.

This demonstration has a real-world weather application that many Central Texans have dealt with recently, hail.

Using the blow dryer, we’re recreating the updraft within a thunderstorm. Our ping pong balls represent the water droplets and hail stones that are also in the storm.

During the life cycle of a thunderstorm, the updraft carries water droplets high up in the atmosphere where they freeze. These ice pellets may fall and melt some, only to be picked up by the updraft once again. As this takes place, the hailstone grows.

Very strong thunderstorms also have very strong updrafts, capable of suspending hail stones that can be the size of baseballs or larger within the upper levels of the storm.

Once the hail becomes too large to be supported by the updraft, gravity takes over and the stone falls back to earth where it can cause a lot of damage.

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