AP Physics AP Physics Rotational Motion & Gravitation 1 — Questions and Answers
Question 1: A solid disk of mass M and radius R rotates about its center. What is its moment of inertia?
- MR²
- (1/2)MR² (Correct answer)
- (2/3)MR²
- (2/5)MR²
Correct answer: (1/2)MR²
The moment of inertia of a solid disk about its central axis is (1/2)MR².
Question 2: A net torque of 12 N·m acts on an object with a moment of inertia of 4 kg·m². What is the angular acceleration?
- 48 rad/s²
- 0.33 rad/s²
- 3 rad/s² (Correct answer)
- 8 rad/s²
Correct answer: 3 rad/s²
By Newton's second law for rotation, α = τ/I = 12/4 = 3 rad/s².
Question 3: Which quantity is conserved when no net external torque acts on a system?
- Kinetic energy
- Linear momentum
- Angular momentum (Correct answer)
- Torque
Correct answer: Angular momentum
Angular momentum L = Iω is conserved when the net external torque on a system is zero.
Question 4: A planet orbits the Sun in an elliptical path. According to Kepler's second law, what remains constant?
- Orbital speed
- Distance from the Sun
- Area swept per unit time (Correct answer)
- Gravitational force
Correct answer: Area swept per unit time
Kepler's second law states that a planet sweeps equal areas in equal times, reflecting conservation of angular momentum.
Question 5: The gravitational force between two masses is F. If the distance between them is doubled, the new force is:
- 4F
- 2F
- F/2
- F/4 (Correct answer)
Correct answer: F/4
Newton's law of gravitation gives F ∝ 1/r², so doubling r reduces F by a factor of 4.
Question 6: A wheel starts from rest and reaches 60 rad/s in 10 s with constant angular acceleration. What is α?
- 600 rad/s²
- 0.17 rad/s²
- 6 rad/s² (Correct answer)
- 70 rad/s²
Correct answer: 6 rad/s²
Using ω = ω₀ + αt: α = (60 − 0)/10 = 6 rad/s².
AP Physics AP Physics Rotational Motion & Gravitation 1
A solid disk of mass M and radius R rotates about its center.
What is its moment of inertia?