Principle Demonstrated: Bernoulli’s Law

Va. SOL:

PH.7   The student will investigate and understand properties of fluids.

            … f) Bernoulli’s principle.

National Standards:

http://www.nap.edu/readingroom/books/nses/html/6e.html#ps

Topic/Concept

Conceptual approach to Bernoulli’s Principle.

Safety Considerations

        Before turning on the leaf blower look in the direction the blower will be pointing. Don’t aim the leaf blower towards any loose posters on the wall or towards anything hanging from the ceiling.

Materials:

• Leaf blower
• Foot long rod that fits the tube of a roll of toilet paper. A broomstick or 1 foot long piece of 1" diameter PC pipe works.
• 1 roll of toilet paper

Presentation

When doing a demonstration I like to have the students do three steps as part of the demonstration.
  I.   Observe
  II.  Infer
  III. Calculate

Observe the demonstration. If possible I'll have the students be a part of the demonstration.
Infer the concepts behind the demonstration
Calculate: when possible have something to calculate or even better to measure and calculate.

Now for this demo's details.

• Loosen the first couple of sheets of toilet paper so it will unravel easily.
• Put the toilet paper on the rod.
• Hold it up in the air such that the sheets are parallel to the ground.
• Tell the student you are going to turn on the leaf blower. Raise the toilet paper until is just underneath the air stream. You know when to stop lifting because the toilet paper will begin to fly off the roll.

How the physics is demonstrated

Bernoulli’s principle says that air that is moving fast compared to the surrounding air creates a lower pressure. The air moving over the top of the toilet paper is moving faster than the air underneath the toilet paper. This creates a lower pressure above the toilet paper. The high pressure lifts the toilet paper up into the air steam of the leaf blower.

Construction  and Tips Regarding the Demonstration

This presentation is quick, simple and gets more than one student involved. If you have the resources you can redesign a leaf blower just for the purpose of building a quicker "T.P.'ing machine". Get a point roller and slide a roll or 2 of toilet paper on it. Use duct tape, (Gorilla™ tape would be better,) To attach the roller to the end of the leaf blower's air tube. You may need some wood to prop it at the right height and to keep the roller level.

Extension

If the wind speed is 75 mph over the top of the tissue paper then how would a strip of 50 squares be lifted? Better yet, measure the air speed of your leaf blower and use this for this calculation.