Bernoullis Principle

Physics, Engineering:	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

Short Explanation

Bernoulli's Principle states that 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, the net force will cancel out, meaning that the only thing left affecting the ball is the force of gravity. If we use the blower, however, then the air above the ball is no longer pushing straight 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.

The bottle helps to get across how useful this knowledge is. Even though it has more mass to it, 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.

Full Explanation

In fluid dynamics, we can show that, If you have a fluid with velocity v moving past another fluid with a substantially lower velocity, that the amount of pressure that it exerts on any suspended objects perpendicular to its motion is lower than what the slower fluid exerts. The equation we will look at is:

Bernoulli's Equation:	1/2 v^2 + P/d = C
v=Velocity, P=Pressure, d=Density	C=Constant, in units (m/s)^2

In this equation, we have three variables that can be changed: the velocity, the pressure and the density. How those three change will correspond to a constant C. In our case with the blower, we will only be looking at two of the variables, the velocity and the pressure. When we adjust the velocity of the air, we create a stream of it which is moving quickly. This stream of air, since it has an increased velocity, must now adjust either in pressure or density in order to keep our constant C the same. The air pressure around us changes easily, whereas the density of the air around us is hard to adjust. This results in a change of how much pressure the moving stream of air applies perpendicular to itself. By using this equation, we now see that as the velocity of the fluid increases, the pressure needs to decrease in order for the constant to stay the same.

When we lift the ball into the stream of moving air, the top of the ball is placed inside of the high velocity, low pressure area while the bottom is still in the low velocity, high pressure area around it. The fast moving air applies much lower pressure on the ball, which is then lifted into this low pressure area by the high pressure air below it. However, the ball cannot be lifted above the low pressure zone, due to the counter force of gravity. The ball is experiencing an upward force of lift, but it still is affected by the downward force of gravity. If the ball rises too high in the stream, then the net force on it will point downward. Likewise, if the ball drops low, then the net force on it will be upward. This is why the ball will stay above the ground, bobbing slowly in the air stream as it gets pushed by the air all around it.

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. We often hear the statement that it is the difference in air pressure on the sides of a plane's wings that creates lift. However, this difference is not caused directly by the shape of the wing. 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. This means that you could think of the wing of the plane cutting through the air much like a knife cutting through water. The knife separates the water as it slides through, but the water meets again right behind the knife. Likewise, the air traveling over the wing must re-meet the same part of the air traveling under the wing. The top of the wing is designed to have a longer surface, the air is forced to have a higher velocity, and therefore exert less pressure on the wing, 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.