UseMathematics Activity
Air Pressure
First build the particle model of gas pressure. Then use that model to explain why fast-moving air can push on and hold up a ball.
Today’s phenomenon
Watch the video, then answer the question: Why does this ball not fall out of the air stream?
Video: Ball in a fast air stream
Why do you think this ball does not fall out of the air stream once he gets it going?
Kinetic molecular theory: what is a gas doing?
Watch the video, then select only the statements that apply to an ideal gas.
Video A: Kinetic molecular theory of gases
Select all that apply to an ideal gas
- Look for statements that describe motion, collisions, and average speed.
- Be suspicious of statements that say gas particles sit still or stick together easily.
- For ideal gases, attractions between particles are not the main part of the model.
In your own words: What causes gas pressure?
How do number of particles, temperature, and volume change pressure?
The pressure reading is a measure of the pressure the container experiences from the gas inside it.
Use the buttons inside each one to set the model.
The sim stays visible beside the mini-questions, so you do not need to scroll back and forth.
Use speed, collisions, and wall hits.
Gas model
Mini-question 1: What happens when volume changes?
The pressure reading is a measure of the pressure the container the gas is in experiences from the gas inside it. Look at and compare the particles in the low-volume and high-volume settings. Are the particles moving faster, slower, or the same speed? Explain why pressure is larger in a smaller container than in a bigger one.
Particle explanation for volume:
Mini-question 2: What happens when temperature changes?
Keep the number of particles and the container size the same. Compare low temperature to high temperature. Look for whether the particles move faster or slower. Then connect particle speed to how hard and how often particles collide with the container walls.
Particle explanation for temperature:
Mini-question 3: What happens when the number of gas particles changes?
Keep temperature and volume the same. Compare a container with fewer gas particles to a container with more gas particles. Look for whether the particles themselves are faster or whether there are simply more particles available to collide with the walls.
Particle explanation for number of particles:
- For each mini-question, ask: did the particles get faster, or did something else change?
- Then ask: would that make wall collisions happen more often, less often, harder, or softer?
- Use the collision change to explain the pressure reading.
What changes when air becomes a fast stream?
Static air still has molecules moving in all directions. The overall motion cancels out, so the air does not flow across the room. Moving air has many particles sharing the same overall direction.
Observe molecules moving in many directions. The overall air does not travel as one stream.
Let the model run for about 1 minute. Watch the blue stream and the side boundaries.
Compare red entering particles with green leaving particles.
Observation with blower off: How are the molecules moving?
| After about 1 minute with blower on | Your recorded value |
|---|---|
| Entering stream - red count and percent | |
| Leaving stream - green count and percent | |
| Which happened more? |
- Look at the blue stream. Which direction are most blue particles carried?
- If a particle enters the blue stream from the side, what can happen to its motion?
- Use the counts as evidence. Do not assume the result before reading the numbers.
Explain the interesting pattern: Why might more air enter the fast stream from the sides than leave it sideways?
Explain the difference between static air and moving air. (Static air is air that is not moving in a specific direction, like the air outside the blue moving air column.)
Why can a ball float in a fast stream of air?
Use the Part 3 simulator as your model: compare the region outside the air stream to the region inside the air stream.
Video: Another ball example
Optional support: pressure in moving fluids
Observation: What did the ball do?
Outside the air stream
This is the surrounding air. In the simulator, particles from this region can enter the stream from the sides.
Inside the air stream
This is the fast-moving air inside the air stream.
Use the Part 3 simulator
| Question | Choose the best answer |
|---|---|
| If the ball drifts partly out of the air stream, what does the simulator suggest can happen near the side of the stream? | |
| What upward force helps keep the ball from falling? | |
| Why does the ball stop rising instead of flying upward forever? |
- Think about the ball drifting slightly left or right. Which side would have more surrounding air available to enter the stream?
- What direction does the fast air stream push on the bottom of the ball?
- What has to become balanced for the ball to hover at one height?
Complete explanation: Why does the ball stay in the air stream instead of falling or flying away?
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