Whelmer #12: Bernoulli Cans
Description: Two empty soft drink cans are placed on several drinking straws. Air pressure forces the cans to roll toward each other. Science process skills:
K-4:
Content topics:
This is a simple activity. However, practice this activity before presenting it to your students. For the best results, the activity should be presented on a smooth and level table top. Place 23 straws on the table parallel to each other, about 1 cm apart. Place the cans upright on the rank of straws. The cans will be able to roll freely, back and forth. Position the cans approximately 5 cm. apart. Using the remaining straw, blow between the cans. The cans roll towards each other, colliding with a clang. Presentation: Although this activity is fairly simple, if affords several opportunities to model prediction and analytical thinking. First demonstrate how easy it is for the cans to roll back and forth on the straws. Explain to your students how you are going to blow between the cans and ask them to predict what is going to happen. Many students will suggest that the cans will roll apart due to the additional air you are forcing between the cans. Ask students, "Other than the straws, what is touching the cans?" (Answer: air). Is blowing between the cans going to increase the air pressure momentarily by adding more local air, or decrease the pressure momentarily by "knocking" some of the local air out of the way? If students suggest that the cans will roll towards each other, ask them to explain their prediction. What are their prior experiences that would allow them to make such a prediction? (Answer: wind blowing over a sheet of paper and "lifting" it; something getting "pulled" into a current). Allow students to blow between the cans, causing the cans to come together. Direct them to make a top-view drawing of the two cans and use arrows to indicate the forces of air pressure. They should indicate that the air pressure between the cans decreases and the air on the outer sides of the cans forces them together. Content: In 1738, a Swiss mathematician Daniel Bernoulli studied the relationship between the pressure and velocity of a fluid. The Bernoulli Principle states that the pressure of a liquid decreases as its velocity increases. That principle applies to the two cans. As the velocity of the air between the two cans increases (being blown away), 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 to the area of lower pressure. Make certain that your students understand that the air pressure on the outer sides did not increase, rather it was the decrease in pressure between the cans that allowed the cans to roll towards each other. The cans were not "sucked" together. They were pushed together. Assessment: Type: individual or pairs. Content/Process: Bernoulli's Principle. Age/Level: intermediate or higher. Whelmer #12 can be used as the assessment following Whelmer #28: T. P. Away! Analysis of the drawing of the forces acting on the cans should indicate an understanding of unbalanced forces and the impact of a reduction in air pressure. A rubric may be used or a qualitative designation (e.g. incorrect, partially complete, complete) can serve as the assessment. If this Whelmer is used independently of other Whelmers, the assessment can be embedded in the activity. When asking for predictions and prior experiences upon which the predictions were made, the teacher could note the appropriateness of the experiences presented and any underlying misconceptions on a prepared class list of students ("clipboard cruising"). Notes: |
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