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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.

  1. 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.

  2. 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.

  3. 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.

  4. 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.

  5. Question 5

    An ideal gas is enclosed in a cylinder with a movable piston. The gas initially has volume V0V_0 and density ρ0\rho_0. If the piston is moved to compress the gas to a new volume of 13V0\frac{1}{3}V_0 while keeping the mass constant, which expression represents the new density?

    3ρ03\rho_0

    13ρ0\frac{1}{3}\rho_0

    9ρ09\rho_0

    19ρ0\frac{1}{9}\rho_0

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    Answer: (A)

    Density is defined as mass per unit volume (ρ=m/V\rho = m/V). Since mass is constant and volume decreases by a factor of 3, the density must increase by a factor of 3 (inverse proportionality).

  6. Question 6

    Planet A has mass MM and radius RR. Planet B has mass 4M4M and radius 2R2R. If both planets are uniform spheres, which expression correctly predicts the density of Planet B (ρB\rho_B) in terms of the density of Planet A (ρA\rho_A)?

    ρB=0.5ρA\rho_B = 0.5\rho_A

    ρB=2ρA\rho_B = 2\rho_A

    ρB=4ρA\rho_B = 4\rho_A

    ρB=0.25ρA\rho_B = 0.25\rho_A

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    Answer: (A)

    Density ρ∝M/R3\rho \propto M/R^3. For Planet B, the mass increases by factor 4 and radius by factor 2, so ρB∝4M/(2R)3=4M/8R3=0.5(M/R3)\rho_B \propto 4M/(2R)^3 = 4M/8R^3 = 0.5(M/R^3).

  7. Question 7

    A copper wire has length LL, cross-sectional area AA, and density ρ\rho. The wire is stretched until its length is 2L2L. Assuming the volume of the copper remains constant during stretching, what is the new density of the wire?

    ρ\rho

    0.5ρ0.5\rho

    2ρ2\rho

    0.25ρ0.25\rho

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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.

  8. Question 8

    An ideal fluid flows through a horizontal pipe that widens from a radius of rr to a radius of 2r2r. If the density of the fluid in the narrow section is ρ1\rho_1, which expression represents the fluid density ρ2\rho_2 in the wide section?

    ρ2=ρ1\rho_2 = \rho_1

    ρ2=0.25ρ1\rho_2 = 0.25\rho_1

    ρ2=0.5ρ1\rho_2 = 0.5\rho_1

    ρ2=4ρ1\rho_2 = 4\rho_1

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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.

  9. Question 9

    A container is filled with a volume VV of liquid A (density ρ\rho) and a volume VV of liquid B (density 3ρ3\rho). If the liquids mix completely and their volumes are additive, what is the density of the resulting mixture?

    2ρ2\rho

    1.5ρ1.5\rho

    4ρ4\rho

    3ρ3\rho

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    Answer: (A)

    Total mass is ρV+3ρV=4ρV\rho V + 3\rho V = 4\rho V. Total volume is 2V2V. The new density is total mass divided by total volume: 4ρV/2V=2ρ4\rho V / 2V = 2\rho.

  10. Question 10

    A chemist mixes a mass MM of fluid X (density ρ\rho) with a mass MM of fluid Y (density 3ρ3\rho). Assuming the volumes are additive, which expression represents the density of the resulting mixture?

    1.5ρ1.5\rho

    2.0ρ2.0\rho

    0.75ρ0.75\rho

    3.0ρ3.0\rho

    Show answer

    Answer: (A)

    Total mass is 2M2M. Volume of X is M/ρM/\rho, Volume of Y is M/3ρM/3\rho. Total Volume is 4M/3ρ4M/3\rho. Density = 2M/(4M/3ρ)=1.5ρ2M / (4M/3\rho) = 1.5\rho.

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.

Review Unit 8 before you start

Other AP Physics 1 unit tests