Bernouli Principle

Submitted by Jeanelle Day, Council for Elementary Science International

Standards (NSES)

K-4th:

  • Use data to construct reasonable explanations. (A.1.4)
  • The position and motion of objects can be changed by pushing or pulling.    
  • The size of the change is related to the strength of the push or pull. (B.2.3)

5th-8th:

  • Unbalanced forces will cause changes in the speed or direction of an object’s motion. (B.2.3)
  • Energy is a property of many substances and is associated with mechanical motion. Energy is transferred in many ways. (B.3.1)

Engagement

Ask the following questions to elicit responses:

  • What is air pressure?
  • Have you flown in an airplane?
  • How does an airplane fly if it weighs almost 100,000 pounds?

Explore

Do the following activity:

  1. Take 4 1/4 x 11 inch pieces of paper. Hold the short side toward you.
  2. Measure and cut a 5-inch slit lengthwise down the center of the paper.
  3. Mark edges at end of slit on opposite sides. Cut a 1-inch slit in from each edge at the marks.
  4. Fold in the sides below the slits toward the center; they will overlap.
  5. Use tape to attach the two folded sides together.
  6. Fold the bottom edge up twice, 1 inch at a time. Fasten with paper clips.
  7. Fold one flap toward you and the other away from you.
  8. Throw "Whirlybird" straight up in the air to make it fly downward.

How do you make the Whirlybird spin in the opposite direction? How do you make it drop or descend more slowly?

  • Teacher Note: Why does this work? - The spinning effect is due to the action of air on the wings as it rushes past the dropping Whirlybird. The longer the wings, the slower the drop because of the uplift on the greater wing area. The more paperclips, the fast the drop and spin because of the greater weight. Flipping the wings will cause it to spin in the opposite direction.

Whirlybird activity adapted from http://www.ied.edu.hk/apfslt/v3_issue2/tytler/tytler4.htm

Explanation

  1. Introduce the following terminology (definitions from
    http://www.brown.edu/Departments/EEB/EML/background/principles_flight.htm):
    • Lift - a positive force caused by the difference in air pressure under and above a wing. The higher air pressure beneath a wing creates lift, and is affected by the shape of the wing. Changing a wing's angle of attack affects the speed of the air flowing over the wing and the amount of lift that the wing creates.
    • Drag - the resistance of the air to anything moving through it. Different wing shapes greatly affect drag. Air divides smoothly around a wing's rounded leading edge, and flows neatly off its tapered trailing edge...this is called streamlining.
    • Weight - the force that causes objects to fall downwards. In flight, the force of weight is countered by the forces of lift and thrust.
    • Thrust - the force that propels an object forward. An engine spinning a propeller or a jet engine expelling hot air out the tailpipe are examples of thrust. In bats, thrust is created by muscles making the wings flap.
  2. Ask the following questions: Do birds us the same principles of flight as planes? Show the video clip to demonstrate how a bat flies: http://www.brown.edu/Departments/EEB/EML/videos.htm
  3. Introduce Bernouilli's Principle: As the velocity of a fluid increases, the pressure exerted by that fluid decreases. Airplanes get their lift through Bernouilli's Principle.

Expansion

Why do the cans clang together? (adapted from http://www.mcrel.org/whelmers/whelm12.asp)

  1. Place 23 straws on the table parallel to each other, about 1 cm apart.
  2. Place the cans upright on the straws. Position the cans approximately 5 cm apart.
  3. Using a straw, blow between the cans. The cans roll toward each other colliding with a clang.

Teacher Note: Why does this Happen? - As the velocity of the air between the two cans increases, the pressure the air it applies to the inner sides of the cans decreases. That allows the air on the opposing sides of the cans to push the cans towards the area of lower pressure. (The air pressure on the outer sides of the cans did not increase, instead there was a decrease in the pressure between the cans that allowed the cans to roll towards each other. The cans were not sucked together, they were pushed together). This is known as an inverse relationship.

A video example of Bernouilli's Principle at work can be found at http://www.youtube.com/watch?v=AUyczZ3EiZg.

 

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