Bernoulli Effect

Age

Elementary School, Middle School, High School

Format

Stage Show, Hands-on

Materials

   Blower
   Blower hose
   Electrical Extension cord
   Beach Ball
   2-liter pop bottle, empty (with rounded bottom)
   2-liter pop bottle, with some water (with rounded bottom)
   Roll of one-ply toilet paper
   Wooden Dowel (with fringe on one end) 

Safety Precautions

Science Theatre demonstrators must keep the safety of themselves and their audience in mind at all times. All Science Theatre demonstrators must have read through the Safety Training page. The ST Safety Box with first aid kit, fire extinguisher, etc. should always be available to demonstrators. Always wear safety gloves, glasses, and a labcoat if handling chemicals; always perform potentially dangerous demonstrations at a safe distance from the audience; and always keep a very close eye on any volunteers you call from the audience. Make sure objects are not blown into the audience.

Preparation

Obtain all the materials listed above. Attach the blower hose to the air output of the blower. Ensure the blower has power and is operating correctly. Inflate the beach ball. Have on hand the two 2-liter pop bottles (one w/ water one w/o water). Have on hand the wooden dowel and toilet paper.

Demonstration

Part 1: Turn the blower on low power and while holding the hose at a sharp upward angle, find the position at which the beach ball will float. The wooden dowel can be used to show the moving air as the fringe will point in the direction of air flow. Notice the moving air on top of the ball, but no moving air under it. The dowel can also be used to show that there is nothing holding the ball in the air (ie. hidden strings, etc.)

Part 2: Continue to demonstrate the principle as demonstrated in part 1, but this time use the empty 2-liter bottle.

Part 3: Continue to demonstrate the principle as demonstrated in part 1, but this time use the 2-liter bottle with water in it. At this time it also works best to have the blower on high power.

Part 4: Place the toilet paper roll on the wooden dowel and hold the toilet paper in front of the blower (on high power). The toilet paper will begin to unravel and fly off the roll. This can be directed into the audience if desired.

What to Say

Part 1: Ask the kids if they think you can make the beach ball float in the air. Allow them to answer, then continue on to demonstration part 1. Explain that the air is moving faster on the top of the ball then on the bottom of the ball and that this means there is less pressure on the top and more pressure on the bottom. This causes lift and the ball stays aloft. Be sure to show the moving air on the top versus the still air on the bottom by using the fringe end of the wooden dowel.

Part 2: Start demonstration part 2. You can re-iterate the general concepts that you described in part 1, again showing the moving air versus the non-moving air. Again, explain that the fast moving air creates low pressure.

Part 3: Start demonstration part 3. Ask the kids if they think you can make a 2-liter pop bottle float if it has water in it and is heavy! Allow the kids to answer. Show the kids that it is in fact possible to make the water weighted bottle float. Remember to use the higher blower setting. You can again explain what is happening (high velocity = low pressure = lift). Be excited that the lift is strong enough to hold the bottle up!!!!

Part 4: Start demonstration part 4. Tell the kids that you are going to do something fun with Bernoulli's Principle. How about making a HUGE streamer of toilet paper!!!! Start by blowing the streamer to the side of the stage and then you can slowly move the paper so it is directed over the audience;. if desired. The paper is light and harmless so there is no inherent danger in this demo.

An airplane's wing forces air to move very quickly over the top of it. Therefore the pressure of the air above the wing is lower than the air below the wing, by Bernoulli's principle - so airplanes can fly!

Why It Is: This simplistic explanation of Bernoulli's Principle stems from Bernoulli's equation which states that the sum of all forms of energy in a fluid flowing along an enclosed path is the same at any two points in that path. This means that a change in pressure will cause a change in velocity. This is demonstrated by an airplane wing, where the change in pressure due to the curved surface changes the velocity at which the air flows because the sum of the velocity and pressure must be a constant.

1/2*Velocity2 + Pressure/Density = Constant

The above equation shows that as the pressure increases, the velocity must decrease in order to keep the sum truly a constant. It can also be said that as the velocity increases, the pressure must decrease in order to keep the sum at a constant. The above equation excludes the (gravity x height) term that should be included in the sum.

It can easily be seen that an airplane can generate some lift or upward force due to the higher pressure on the bottom of a wing versus a lower pressure on the top. This pressure is caused by the shape of the wing, forcing the increase in velocity and decrease in pressure. Please note that it is NOT caused by the air having to travel a longer distance over the curved side in the same time interval as the bottom of the wing. The reason it is traveling faster is actually due to conservation of mass (oddly enough). Note in the image below the two different air streams. The upper stream constricts as it flows over the airfoil. Due to conservation of mass laws, the air must increase its velocity over the airfoil and thus, by Bernoulli's Principle, exerts less pressure on the top of the wing. Conversely, the lower stream expands causing the air velocity to decrease and thus increasing the pressure.

Real Life Examples

Airplanes, car spoilers, hydrofoils, inside carburetors