PPTBernoulli’s Principle

Developer Notes

Version / Date / Who / Revisions
01 / 2004/07/02 / Sc / Initial version
02 / 2004/07/06 / dk /
  • Added presentation, examples, and descriptions of stations

Goals

  1. Students will understand that fluids with higher velocity have lower pressure (and vice versa).

Concepts & Skills Introduced

Area / Concept
Physics / Bernoulli’s Principle

Standards Addressed

Time Required

Warm-up Question

Presentation

The goal of this activity is just to show that fluids with higher velocities have lower pressure. Do the stations first, then discuss the results as a class. For the big bag, the trick is to blow in it without closing the end. The air you blow in will draw more air in with it.

Bernoulli’s Principle

Where the velocity of a fluid is high, the pressure is low, and vice versa.

At a molecular level, this is because molecules in still air are bouncing in all directions, whereas molecules in a flow don’t bounce off the surface they are flowing past as much.

There are many practical applications of Bernoulli’s Principle.

  • Garden sprayers - Fluid from the reservoir is drawn into the stream of water because the pressure is lower where the water is moving fast. This is the same for old perfume atomizers.
  • Gas engine carburetors – Air is drawn into the cylinders through the carburetor. The carburetor has a narrowed throat. As the air speeds up through the throat, the pressure is lower, and gas is drawn from the reservoir into the air.
  • Airplane wings – Air flowing over the wings has to travel a longer path than air under the wings, so it flows faster. The faster flowing air has lower pressure, so the wing has lift.
  • Balls – As the ball spins, some air is pulled around with the ball. On the side of the ball that spins forward, the air flowing past the ball and the air flowing with the ball interfere, causing slower flow. On the side of the ball that spins rearward, the two join together to create faster airflow, so the ball curves in the direction of the spin.
  • Waves – Waves get larger in strong winds because the air across the tops of the waves flows faster than the air in the troughs, so the pressure on the peaks is less and the waves get larger.
  • Race cars – Race cars are designed so that the air flowing past the car has a narrowed opening under the car. This causes the air under the car t o flow faster than the air over the car, creating a downward force and better traction on the road.
  • Roofs and windows – Roofs and windows blow off of houses during storms because the pressure of the stationary air inside the house is greater than the pressure of the fast moving air outside the house.
  • (I’m not sure of this one. When you cough, is it the momentum of the air transferring to the phlegm to make it move, or is it the low pressure of the fast-moving air that draws the phlegm?)

Assessment

Writing Prompts

Relevance

Answers to Exercises

Answers to Challenge/ extension

Background

Problem

Materials

Procedure

Stations

Give each student a flexible straw to carry with them through the stations.

  1. Ping pong ballstraw
  2. Materials – ping pong ball, flexible straw. Bend the straw at a right angle.
  3. Procedure - Hold the ping pong above the short end of the straw and blow through the long end. See how high you can blow the ball. Notice that the ball does not blow away, but stays above the straw.
  4. Funnel & ping pongball– skip this one? Very similar to straw and PP ball, but unsanitary.
  5. Materials– ping pong ball and funnel
  6. Place the ball in the funnel. Blow through the funnel. What happens?
  7. Bent index card & straw
  8. Materials – index card, straw. Bend the index card into a bridge as shown.
  9. Procedure - Blow under the index card with the straw and see what happens. What happens to the bridge as you blow under it? Can you blow it away?
  10. Spool
  11. Materials – spool, index card, straight pin. Push the pin through the center of the index card.
  12. Procedure - Place the index card on the spool so that the pin goes through the hole in the spool. Blow through the bottom of the spool. Can you blow the index card off? Turnthespool so that the index card is on the bottom and blow through the spool. What happens?
  13. Paper
  14. Materials – sheet of regular paper
  15. Procedure – Hold the paper so that it is just below your mouth. Blow over it. What happens?
  16. Hanging balls & straw
  17. Materials– ping pong balls, dental floss, straw. Suspend the ping pong balls with the floss so that they are about an inch apart. (This could also be done with soft drink cans.)
  18. Procedure blow between the ping pong balls with the straw. What happens?
  19. Atomizer
  20. Materials – straw, container of water, small tube
  21. Procedure – Place the small tube in the water. Blow over the end of the tube with the straw. What happens?
  22. Airplane wing?
  23. Big bag
  24. Materials – big bag?
  25. Procedure – How many breaths will it take you to blow up the bag?

Summary

  1. Why does the ping pong ball stay above the straw instead of blowing away?
  2. Why does the ping pong ball stay in the funnel?
  3. Why does the index card not blow away?
  4. Why doesn’t the index card blow off the spool?
  5. Why does the paper rise when you blow above it?
  6. Why do the ping pong balls move toward each other?
  7. Why does the water rise through the tube?
  8. Why does the airplane wing rise?

Reading

Exercises

Challenge/ extension

Glossary

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