AP Physics 1 Unit 8 practice questions
Try 10 multiple-choice questions from Unit 8 here. Pick an answer to see why it's right or wrong, then take the timed test above for a full AP-style set.
Question 1
A solid object of total volume V₀ and density ρ₀ floats in a liquid of density ρ_L, where ρ_L > ρ₀. Which of the following expressions gives the volume of the object that remains above the surface of the liquid?
V₀(1 - ρ₀/ρ_L)
V₀(1 - ρ_L/ρ₀)
V₀(ρ₀/ρ_L + 1)
V₀(ρ_L/ρ₀ - 1)
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Answer: (A)
Applying Archimedes' principle and volume conservation.
Question 2
A sphere of volume V and density ρ_S is completely submerged in a liquid of density ρ_L and held stationary by a vertical string. If ρ_S > ρ_L, which of the following expressions gives the magnitude of the tension in the string?
Vg(ρ_S - ρ_L)
Vg(ρ_L - ρ_S)
Vg(ρ_S + ρ_L)
Vg(ρ_S + ρ_L)/2
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Answer: (A)
Analyzing the forces in equilibrium using Newton's second law.
Question 3
A small object of mass m and volume V is released from rest at the bottom of a tank filled with a fluid of density ρ. The density of the object is less than ρ, and drag forces are negligible. Which expression gives the magnitude of the object's initial upward acceleration?
g(ρV/m - 1)
g(1 - ρV/m)
g(ρV/m + 1)
g(m/ρV - 1)
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Answer: (A)
Applying Newton's second law with buoyant and gravitational forces.
Question 4
A uniform cube of side length L floats at the interface between two immiscible fluids. The top fluid has density ρ₁ and the bottom fluid has density ρ₂. If the cube is submerged such that exactly half of its volume is in each fluid, which expression gives the mass of the cube?
L³(ρ₁ + ρ₂)/2
L³(ρ₂ - ρ₁)/2
L³(ρ₁ + ρ₂)
L³(ρ₂ - ρ₁)
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Answer: (A)
Summing the buoyant forces from both displaced fluid volumes.
Question 5
An ideal gas is enclosed in a cylinder with a movable piston. The gas initially has volume and density . If the piston is moved to compress the gas to a new volume of while keeping the mass constant, which expression represents the new density?
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Answer: (A)
Density is defined as mass per unit volume (). Since mass is constant and volume decreases by a factor of 3, the density must increase by a factor of 3 (inverse proportionality).
Question 6
Planet A has mass and radius . Planet B has mass and radius . If both planets are uniform spheres, which expression correctly predicts the density of Planet B () in terms of the density of Planet A ()?
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Answer: (A)
Density . For Planet B, the mass increases by factor 4 and radius by factor 2, so .
Question 7
A copper wire has length , cross-sectional area , and density . The wire is stretched until its length is . Assuming the volume of the copper remains constant during stretching, what is the new density of the wire?
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Answer: (A)
Density is an intensive property of the material. Since the mass and volume of the copper remain constant during the stretching process, the density does not change.
Question 8
An ideal fluid flows through a horizontal pipe that widens from a radius of to a radius of . If the density of the fluid in the narrow section is , which expression represents the fluid density in the wide section?
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Answer: (A)
An ideal fluid is defined as incompressible, meaning its density remains constant regardless of changes in pipe geometry or flow speed.
Question 9
A container is filled with a volume of liquid A (density ) and a volume of liquid B (density ). If the liquids mix completely and their volumes are additive, what is the density of the resulting mixture?
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Answer: (A)
Total mass is . Total volume is . The new density is total mass divided by total volume: .
Question 10
A chemist mixes a mass of fluid X (density ) with a mass of fluid Y (density ). Assuming the volumes are additive, which expression represents the density of the resulting mixture?
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Answer: (A)
Total mass is . Volume of X is , Volume of Y is . Total Volume is . Density = .
What's on the AP Physics 1 Unit 8 practice test
The standard version takes about 88 minutes and follows the timing and directions of the AP exam, sized to one unit:
- 20 Multiple Choice Questions (40 minutes)
- 1 FRQ – FRQ 1 – Mathematical Routines (24 minutes)
- 1 FRQ – FRQ 2 – Translation Between Representations (24 minutes)
Questions are drawn from 1105 multiple-choice questions and 39 FRQs written for Unit 8, Fluids.
Practice AP Physics 1 Unit 8 FRQs
Answer the unit's free-response questions one at a time and get a score on every part.
- FRQ 1 – Mathematical Routines (13 questions)
- FRQ 2 – Translation Between Representations (4 questions)
- FRQ 3 – Experimental Design (5 questions)
- FRQ 4 – Qualitative/Quantitative Translation (17 questions)
Review Unit 8 before you start
- 8.1 Internal Structure and Density
- 8.2 Pressure
- 8.3 Fluids and Newton's Laws
- 8.4 Fluids and Conservation Laws