Bernoullis Principle

Physics:	Pressure, Bernoulli's Principle
Grade Range:	Elementary School, Middle School, High School
Format:	Hands-on, Stage

This is one of our most commonly performed demonstrations. It is easy to perform, easy to understand, and the topic matter is easily adjusted for all grades and age levels.

Materials

• Shop Vacuum/Blower with Hose
• Ribbon Wand
• Beach Ball
• Round-bottom Plastic Bottle
• Optional: Roll of Toilet Paper

Safety Precautions

Please read the Physical Demonstration section of the Demonstration Safety page before performing this demonstration.

Demonstration

Preparation: Blow up the beach ball. Make sure the hose is in the blower side, and plug it in. Make sure that there is a small amount of water in the soda bottle.

1. Choose a volunteer from the audience and have them hold the ribbon wand. Explain to the audience that you will be making the beach ball fly by using the blower, but you want them to guess if the air will be blowing over or under the beach ball. Take a vote to see how many think the air should blow over or under.
2. Turn on the blower on the "LOW" setting. Hold the hose at an angle slightly above 45 degrees, and lift the ball into the air stream. It will lift off into the air, and it will float! Have the volunteer hold the ribbons first under the ball, and see that the air isn't blowing underneath! Then, have them hold the ribbon wand over the ball to see the ribbons move, and show that the air blows over the ball!
3. Let the volunteer return to their seat. Turn off the blower, and explain the demonstration.
4. Call on a new volunteer, and have them hold the soda bottle in one hand. Turn on the blower on the "HIGH" setting. Holding the hose at about a 70 degree angle, have the volunteer place the rounded side of the bottle into the air stream. It should float in the air! After showing this for a minute, turn off the blower and explain.
5. Optional: To end the show, call up one more volunteer. give them the ribbon wand, and have them hold it sideways, turned towards the audience. put the toilet paper roll on the wand, with the roll over side towards the audience. Turn on the blower on the "HIGH" setting, and aim above the roll a little above a 45 degree angle. The roll will unravel into the audience!

Why This Works

Bernoulli's Principle States: If you have an object inside of a fluid, then the object will move to the part of the fluid that exerts the least amount of pressure upon it. To understand what this means, we first look at the beach ball. Typically, the air is still around the ball, so the air exerts air pressure on all sides of it equally. Because it is equal on all sides, it will cancel out, meaning that the only thing left affecting the ball is gravity. If we use the blower, however, we now have the air above the ball no longer pushing down on it. instead, that air is now moving sideways. The air under the ball is not moving, though, and it is going to push the ball upward, against the force of gravity. The lift, or upward force, is stronger than the force of gravity, so the ball stays suspended in the moving air.

When we are able to, we will include a few diagrams in this explanation to help visualize the science behind it.

The bottle helps to get across how useful this knowledge is. Even though it has more mass to it, and therefore more weight, we can still get it to hover in the air by using this concept. The curve of the bottle's base allows it to float, much like the curve on a plane's wing will allow it to fly. A plane is able to fly thanks to utilization of this concept, along with a lot of important engineering to make the plane able to ascend, descend and turn in the air.

A more thorough explanation: In fluid dynamics, we can show that, as a fluid moves in a direction with a velocity v, that the amount of pressure that it exerts on any objects perpendicular to the motion will decrease. The equation is:

1/2 v^2 + P/d = C, where v=Velocity, P=Pressure, d=Density, and C=Constant, with units of (m/s)^2

By using this equation, we can see that as the velocity of the fluid increases, the Pressure needs to decrease in order for the constant to stay the same. This also helps us to understand one of the more tricky parts of this principle, which is that the reason why this happens is, surprisingly enough, due to the Conservation of Mass. As a plane's wing travels through the air, we are correct in that the air on top of the wing is moving faster than the air under the wing. However, this happens because the air the wing travels through has equal density throughout, and so the air traveling over the wing must re-meet the same part of the air traveling under the wing. Since the top of the wing has a longer surface, the air is forced to have a higher velocity, and therefore exert less pressure on the wing, so to make the full trip in time.

Additional Information

• This is not a typical "hands-on" demonstration, since the blower should not be operated by non-presenters. However, it does go over well at Science Fairs and Festivals!
• This demonstration is a part of the Pressure Show.