Choose a comparison level
Measure pressures at equal heights in connected stationary liquid and test the limits of this rule.
AP Physics 1 · Unit 8 · Fluids
Hydrostatics
Compare connected liquid columns and use their measured levels to find an unknown gas pressure or liquid density.
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Learn by doing
Three ideas you’ll put into motion
Read a manometer
Connect surface-level differences with gas-pressure differences, then calculate and check an unknown pressure.
Balance different liquids
Develop a two-liquid pressure balance, determine an unknown density and design a target pressure setting.
The big idea
The key ideas, at a glance.
Match points in connected liquid
At the same height in the same connected stationary liquid, pressures are equal. Moving downward adds ρgh; moving upward subtracts it.
Include each column above the line
For two liquids open to the same atmosphere, the pressure balance gives ρfhf = ρwhw. The heights belong to the columns above the chosen comparison level.
A quick comparison
Double a gas-pressure difference while keeping the measuring-liquid density and gravity fixed.
The equilibrium surface-level difference becomes twice as large.
Teaching this topic?Classroom fit, AP connections, and support
- Classroom fit
- Use for a concept introduction, guided independent practice, or a projected class discussion.
- Before students begin
- Density, pressure, atmospheric and gauge pressure, and P = P₀ + ρgh. The Pressure lesson prepares these ideas.
- Built-in support
- Prepared scenes, guided prompts, and feedback on lesson questions.
- AP Physics 1 connections
- Unit 8, Topic 8.2: hydrostatic pressure. Use a reference pressure and liquid columns to construct mathematical representations and justify claims from measurements.
- Check for understanding
- Ask students to choose a valid equal-pressure line, distinguish a level difference from a whole column height, and explain why equal height alone is insufficient when the points are in different liquids.
Ready to test your thinking?
The first scene is set up. Start the lesson and explore.