AP Physics AP Physics Rotational Motion & Gravitation

Question 1

A solid disk of mass M and radius R rotates about its center. What is its moment of inertia?

Accepted Answer

(1/2)MR²

Answer

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?

Accepted Answer

3 rad/s²

Answer

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?

Accepted Answer

Angular momentum

Answer

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?

Accepted Answer

Area swept per unit time

Answer

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:

Accepted Answer

F/4

Answer

Newton's law of gravitation gives F ∝ 1/r², so doubling r reduces F by a factor of 4.

AP Practice Test