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AP Physics 1 6.1: Rotational Kinetic Energy

Relate a rigid system’s rotational kinetic energy to its rotational inertia and angular velocity, and compare rotational and translational energy.

Syllabus
Effective Fall 2025
Course
AP Physics 1: Algebra-Based

6.1.A—Describe the rotational kinetic energy of a rigid system in terms of the rotational inertia and angular velocity… question 1

[Maximum number: 1]

(7 points, suggested time 13 minutes)

A rod with a sphere attached to the end is connected to a horizontal mounted axle and carefully balanced so that it rests in a position vertically upward from the axle. The center of mass of the rod-sphere system is indicated with a ⊗, as shown in Figure 1. The sphere is lightly tapped, and the rod-sphere system rotates clockwise with negligible friction about the axle due to the gravitational force.

A student takes a video of the rod rotating from the vertically upward position to the vertically downward position. Figure 2 shows five frames (still shots) that the student selected from the video.
Note: these frames are not equally spaced apart in time.

Figure 2

Figure 2

Use the frames of the video shown in Figure 2 to answer the following questions.

In which frame is the rotational kinetic energy of the rod-sphere system the greatest? Briefly justify your answer.

Figure 3

Figure 3

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