Choose a system boundary that makes energy accounting clear, and decide whether work or nonconservative interactions transfer energy across that boundary.
(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
The rod-sphere system has mass M and length L, and the center of mass is located a distance from the axle, shown in Figure 3.
Briefly explain why the rod and sphere gain kinetic energy, even if Earth is not included in the system.