AP Physics:Air and Fluid Pressure Activity
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From Chapter 7 of AP Physics B
• Understand and apply the relationship between pressure and depth in a fluid.
This activity challenges students to explain very familiar phenomena using the concept of fluid pressure. In the process, even advanced students will have their misconceptions about pressure revealed. This activity can be used as a formative assessment of conceptual understanding or even as "practicum" component on a test or quiz.
- Provide students with a few basic supplies: 5-10 plastic straws, a small cup filled with water.
- Ask students to perform the following three experiments and record their observations as well as a detailed explanation using the concept of pressure.
Experiment 1: With the straw only in air, place a finger over the top of the straw, closing the top end off. Put the bottom end of the straw into the cup of water and record your observations and explanation.
Experiment 2: With the straw open on both ends, put the straw halfway submerged into the water. Place a finger over the top of the straw, closing the top end off. With a finger still over the top, remove the straw from the water and record your observations and explanation.
Experiment 3: Make an extra long straw by connecting two straws together in a chain. Place the straw in a cup of water and attempt to move the water up the straw to your mouth with a single long and constant draw. If you are successful in getting water all the way to the top, then add another straw to your chain and try again. Find the highest straw length where it is no longer possible to move the water to your mouth with a single constant draw. Record your observations and explanation.
- Allow the students to experiment, providing as much autonomy as possible and trying not to influence their observations or explanations.
- After students have completed the experiments, ask them to share their observations by facilitating a class discussion. Go through each experiment one-by-one, with students sharing their observations and then discussing their explanations of the results.
- Use leading questions to correct misconceptions and always return to the experiment when there is disagreement about what actually happened.
- As a final test of their understanding ask students if there is an absolute limit to the height of a usable straw (even if a machine used the straw instead of a person). If so, what would cause this limitation? and how high would the maximum height be?
- Each of these experiments demonstrates the strong influence of air pressure. Students will be tempted to explain the experiments in terms of "vacuums" and "suction." Feel free to forbid the use of these two terms at the start of the lesson.
- When "sucking" water up the straw, be sure that students keep the straws vertical, so that they do in fact reach a limit. Students will reach various limiting heights with their straws. This height is limited by the degree to which the pressure in the straw is lowered below atmospheric pressure. In some sense, this height can be used as a measure of lung strength.
- The absolute limiting height up to which a straw can be used is determined by atmospheric pressure. The pressure caused by the weight of the column of air above the surface of the water can push the water up the straw until the weight of the fluid in the straw matches that of the column of air. Estimates on a numerical value for this height can be performed using the densities of air and water as well as the approximate column height of the atmosphere.
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NIST Reference on Constants, Units, and Uncertainty:
AP Physics B Site from Dolores Gende, AP Central's content advisor for physics since 2004:
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