AP Physics C: Mechanics Unit 2 practice questions
Try 10 multiple-choice questions from Unit 2 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
Two satellites orbit a planet in circular paths. Satellite A has an orbital radius R, and Satellite B has an orbital radius 4R. Which claim correctly compares the orbital period T_B of Satellite B to the period T_A of Satellite A?
T_B = 8 T_A, because the square of the period is proportional to the cube of the radius
T_B = 4 T_A, because the period is directly proportional to the orbital radius value
T_B = 2 T_A, because the period is proportional to the square root of the radius value
T_B = 64 T_A, because the period is proportional to the cube of the orbital radius
Show answer
Answer: (A)
Applying Kepler's third law (T² ∝ r³) requires cubing the radius factor (4³ = 64) and taking the square root to find the period factor (√64 = 8).
Question 2
An astronomer calculates the mass M of a star based on the period T and orbital radius R of a planet in a circular orbit. Which expression represents the correct application of Newton's laws to determine M?
M = (4π²R³) / (GT²), derived by equating gravitational force to centripetal force
M = (GT²) / (4π²R³), derived by equating gravitational force to centripetal force
M = (4π²R²) / (GT²), derived by equating gravitational force to centripetal force
M = (2πR³) / (GT), derived by equating gravitational force to centripetal force
Show answer
Answer: (A)
Setting Newton's law of gravitation (GMm/R²) equal to the centripetal force equation (m(2πR/T)²/R) and solving for M yields this expression.
Question 3
A satellite of mass m orbits Earth in a circular path of radius r with period T. If a new satellite of mass 3m is placed in the same orbit, which claim correctly predicts the new period?
The period remains T, because the orbital period depends only on the central mass and radius
The period becomes T/3, because tripling the mass triples the gravitational force acting on it
The period becomes T√3, because the kinetic energy required for the orbit increases with mass
The period becomes 3T, because the satellite's greater inertia requires more time to complete the orbit
Show answer
Answer: (A)
In the derivation of orbital period, the satellite's mass cancels out (GMm/r² = ma_c), indicating that period is independent of the orbiting object's mass.
Question 4
A planet orbits a star of mass M at a distance R with period T. Which claim correctly predicts the period of a planet orbiting a star of mass 2M at a distance 2R?
2T, because the period is proportional to R^(3/2) and inversely proportional to M^(1/2)
4T, because the period is proportional to R^(3/2) and inversely proportional to M^(1/2)
T, because the effects of doubling both the mass and the radius cancel each other out
T/2, because the period is proportional to M^(1/2) and inversely proportional to R^(3/2)
Show answer
Answer: (A)
Using T ∝ √(R³/M), doubling R introduces a factor of √8 (approx 2.8) and doubling M introduces a factor of 1/√2. The combined factor is √(8/2) = √4 = 2.
Question 5
A block slides down a rough incline with an angle of inclination θ. A student claims that increasing θ will decrease the magnitude of the kinetic friction force acting on the block. Which of the following statements correctly justifies this claim using physical principles?
Correct, because the normal force decreases as the cosine of the angle decreases, reducing the friction force.
Correct, because the parallel component of gravity increases, reducing the contact pressure between surfaces.
Incorrect, because the coefficient of kinetic friction increases to compensate for the change in normal force.
Incorrect, because the normal force is equal to the block's weight, which remains constant regardless of angle.
Show answer
Answer: (A)
Connecting the normal force component to the friction equation justifies the claim. The normal force on an incline is mg cos(θ); as θ increases, cos(θ) decreases, causing both the normal force and the proportional kinetic friction force to decrease.
Question 6
A rectangular brick with face areas A₁ and A₂ (where A₁ > A₂) slides across a horizontal floor. A student claims the magnitude of the kinetic friction force is the same regardless of which face is in contact with the floor. Which reasoning best supports this claim?
The friction force depends on the normal force and coefficient of friction, which are independent of the contact area.
The friction force depends on the normal force and surface area, but the normal force increases when area decreases, so the effects balance out.
The friction force depends on the normal force and coefficient of friction, which both decrease proportionally when contact area increases.
The friction force depends on the normal force and coefficient of friction, which remain constant because the brick's weight is distributed equally across any face.
Show answer
Answer: (A)
Identifying the variables in the friction model supports the claim. The Coulomb model of kinetic friction defines the force as the product of the normal force and the coefficient of friction, neither of which changes based on the macroscopic surface area in contact.
Question 7
Block A slides to the right across the top surface of Block B, which is fixed in place. A student claims the kinetic friction force exerted by Block A on Block B is directed to the right. Which justification correctly supports this claim?
Correct, because the force on Block B forms a third-law pair with the leftward friction force on Block A.
Correct, because the force on Block B must oppose the relative velocity of Block A to maintain equilibrium.
Incorrect, because the force on Block B must oppose the direction of the external force pulling Block A.
Incorrect, because the force on Block B acts in the direction of the relative motion of Block A to the left.
Show answer
Answer: (A)
Applying Newton's third law justifies the direction. Since Block A moves right relative to Block B, the friction on A is to the left. The force on Block B must be equal and opposite, therefore directed to the right.
Question 8
A student pulls a crate across a horizontal floor using a rope angled θ above the horizontal. The student claims that increasing θ (while keeping the crate moving) decreases the kinetic friction force. Which reasoning correctly justifies this claim?
The vertical component of tension increases, which reduces the normal force and the friction force.
The horizontal component of tension decreases, which reduces the force needed to overcome friction.
The coefficient of friction decreases, which reduces the resistance between the crate and the floor.
The contact area with the floor decreases, which reduces the mechanical interaction between surfaces.
Show answer
Answer: (A)
Linking the tension component to the normal force justifies the claim. The upward vertical component of tension reduces the normal force required from the floor (N = mg - Tsinθ), which directly reduces the kinetic friction force (f = μN).
Question 9
Two ideal springs with spring constants N/m and N/m are connected in series. What is the equivalent spring constant of this combination?
N/m
N/m
N/m
N/m
Show answer
Answer: (A)
Connecting springs in series reduces the equivalent stiffness according to the reciprocal sum formula.
Question 10
Three identical springs, each with a spring constant of N/m, are arranged in parallel to support a load. What is the equivalent spring constant of this system?
N/m
N/m
N/m
N/m
Show answer
Answer: (D)
Connecting springs in parallel increases stiffness by summing the individual spring constants.
What's on the AP Physics C: Mechanics Unit 2 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 2277 multiple-choice questions and 23 FRQs written for Unit 2, Force and Translational Dynamics.
Practice AP Physics C: Mechanics Unit 2 FRQs
Answer the unit's free-response questions one at a time and get a score on every part.
- FRQ 1 – Mathematical Routines (7 questions)
- FRQ 2 – Translation Between Representations (3 questions)
- FRQ 3 – Experimental Design (7 questions)
- FRQ 4 – Qualitative/Quantitative Translation (6 questions)
Review Unit 2 before you start
- 2.1 Properties and Interactions of a System
- 2.2 Forces and Free-Body Diagrams
- 2.3 Newton's Third Law
- 2.4 Newton's First Law
- 2.5 Newton's Second Law
- 2.6 Gravitational Force
- 2.7 Kinetic and Static Friction
- 2.8 Spring Forces
- 2.9 Resistive Forces
- 2.10 Circular Motion