AP Physics 1: Algebra-Based, AP exams, 10–15%

Unit 8: Fluids

Unit 8 joined AP Physics 1 in the revised framework. It covers density and pressure, buoyancy through Archimedes' principle, and fluid flow through the continuity equation and Bernoulli's principle. The unit is best learned as an application of Newton's laws and conservation of energy rather than as a new set of formulas.

Position: 8 of 8 units
Published weighting: 10–15% in the official framework
Topics covered: 4 topics in this guide

What is covered

Topics inside this unit.

These topics describe the scope students most often review for this part of AP Physics 1: Algebra-Based. Treat them as a study checklist rather than an exhaustive list of what a single exam form can sample.

Internal structure and density: Density as mass per volume, and how the particle model explains fluid behavior.
Pressure: Pressure as force per area, gauge versus absolute pressure, and how pressure varies with depth.
Fluids and Newton's laws: Buoyant force from Archimedes' principle, and floating and sinking as a force-balance question.
Fluids and conservation laws: The continuity equation from conservation of mass, and Bernoulli's principle from conservation of energy.

Study tips

How to practice Fluids.

Section-specific guidance that follows from how this material is usually tested.

Treat buoyancy problems as free-body diagrams with one extra force rather than as a separate topic.
Derive the depth dependence of pressure once so the relationship is reconstructable under pressure.
Practice continuity and Bernoulli together on pipe problems, since they are usually needed in sequence.

Where this fits

Unit 8: Fluids is 8 of 8 units in the AP Physics 1: Algebra-Based guide, and it carries a published weighting of 10–15% in the official framework. Use the full guide to balance it against everything else you have to review.

Back to the AP Physics 1: Algebra-Based guide Official exam information

Sample Fluids questions

Practice questions, not yet expert-reviewed

Original practice questions written to the published framework for this section. No expert has reviewed them, and they are not taken from any official exam. Try each one before you open the answer.

Question 1Foundational
What is the gauge pressure at a depth of 10 m below the surface of fresh water of density 1000 kg/m^3, using g = 10 m/s^2?
1. A1.0 x 10^4 Pa
2. B1.0 x 10^5 Pa
3. C2.0 x 10^5 Pa
4. D1.0 x 10^6 Pa
Show answer
Answer: B. 1.0 x 10^5 Pa
Gauge pressure is the pressure due to the fluid alone: P = (rho)gh = (1000)(10)(10) = 1.0 x 10^5 Pa. The 2.0 x 10^5 Pa choice adds one atmosphere, which would give the absolute pressure rather than the gauge pressure.
Key insight: Pressure in a static fluid depends on depth, not on the shape or the total volume of the container.
Question 2Foundational
A solid block of volume 3.0 x 10^-3 m^3 is held completely submerged in water of density 1000 kg/m^3. Using g = 10 m/s^2, what is the magnitude of the buoyant force on the block?
1. A0.030 N
2. B3.0 N
3. C30 N
4. D300 N
Show answer
Answer: C. 30 N
Archimedes' principle gives the buoyant force as the weight of displaced water: (1000 kg/m^3)(3.0 x 10^-3 m^3)(10 m/s^2) = 30 N. The block's own density never enters the calculation, only the volume it displaces.
Key insight: The buoyant force equals the weight of the fluid displaced, so two fully submerged objects of equal volume feel the same buoyant force.
Question 3Intermediate
A block floats in fresh water of density 1000 kg/m^3 with 80 percent of its volume below the surface. What is the density of the block?
1. A200 kg/m^3
2. B800 kg/m^3
3. C1000 kg/m^3
4. D1250 kg/m^3
Show answer
Answer: B. 800 kg/m^3
For a floating object the buoyant force equals the weight, so (rho of block)V = (rho of fluid)(0.80V) and the block's density is 0.80 x 1000 = 800 kg/m^3. The 1250 kg/m^3 choice divides by 0.80 instead of multiplying, giving a density larger than water, which would sink.
Key insight: The submerged fraction of a floating object equals the ratio of its density to the fluid's density.
Question 4Intermediate
Water flows steadily at 2.0 m/s through a pipe of cross-sectional area 0.020 m^2. The pipe then narrows to a cross-sectional area of 0.0050 m^2. What is the speed of the water in the narrow section?
1. A0.50 m/s
2. B2.0 m/s
3. C4.0 m/s
4. D8.0 m/s
Show answer
Answer: D. 8.0 m/s
Continuity requires A1v1 = A2v2, so v2 = (0.020)(2.0)/(0.0050) = 8.0 m/s. The 0.50 m/s choice inverts the area ratio, which would mean the fluid slows down as the pipe narrows.
Key insight: Conservation of mass forces an incompressible fluid to speed up wherever the pipe narrows.
Question 5Advanced
A large open tank of water has a small hole in its side 7.2 m below the water surface, and the water level drops only very slowly. Using g = 10 m/s^2, at what speed does water leave the hole?
1. A7.2 m/s
2. B8.5 m/s
3. C12 m/s
4. D72 m/s
Show answer
Answer: C. 12 m/s
Applying Bernoulli's equation between the open surface and the hole, where both are at atmospheric pressure, leaves gh = (1/2)v^2, so v = sqrt(2gh) = sqrt(144) = 12 m/s. The 72 m/s choice reports gh instead of the square root of 2gh, and 8.5 m/s is sqrt(gh), which drops the factor of 2.
Key insight: Efflux speed from a hole matches the speed of an object dropped from the same depth, because both convert the same gh into kinetic energy per unit mass.

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