AP Physics C Mechanics 2.2: Free-Body Diagrams

Represent all external forces on an object or system with a free-body diagram, preserving directions, magnitudes, and the selected boundary.

[Maximum number: 3]

A uniform disk of radius R and mass $m_{\mathrm{d}}$ is attached to a vertical pole by a horizontal axle that passes through the center of the disk. Friction between the axle and the disk is negligible. A lump of clay of mass $m_{\mathrm{c}}$ is attached to the edge of the disk at Point A. The size of the lump of clay is small compared with the radius of the disk. A horizontal string is connected from the pole to the edge of the disk at Point A. The string makes an angle $\theta$ with the line between Point A and the axle, as shown in Figure 1.

On the following representation of the clay-disk system, draw and label the external forces (not components) exerted on the system. Each force must be represented by a distinct arrow that starts on, and points away from, the point at which the force is exerted on the system.

Figure for Question 2.2.B—Describe the forces exerted on an object or system using a free-body diagram question 1 — AP Physics C: Mechanics

For drawing and appropriately labeling separate gravitational forces that are exerted on the 1 point disk and the lump of clay and exerted at the correct locations Scoring Note: Drawing a downward gravitational force exerted on the clay-disk system at the correct location can earn this point.

For drawing and appropriately labeling a leftward force exerted on the system at Point A

1 point

For drawing and appropriately labeling a force directed up and right that is exerted on the

1 point
system at the axle, and no extraneous forces are present

Example Responses

Figure for Answer to Question 2.2.B—Describe the forces exerted on an object or system using a free-body diagram question 1 — AP Physics C: Mechanics

Scoring Note: Examples of appropriate labels for the gravitational force include $F_{\mathrm{G}}, F_{g}$ , $F_{\text {grav }}, W, m g, M g$ , "grav force," " F Earth on disk," " F on disk by Earth," $F_{\text {Earth on Disk }}$ , $F_{\mathrm{E}, \text { Disk }}$ , and $F_{\text {Disk, } \mathrm{E}}$ . The labels G or g are not appropriate labels for the gravitational force.

Scoring Note: Examples of appropriate labels for the normal force from the axle include $F_{N}$ , $F_{\text {axle }}, N$ , "normal force," and "axle force."

Scoring Note: Examples of appropriate labels for the tension force include $F_{t}, F_{\mathrm{T}}, T$ , $F_{\text {string }}$ , and "Force from string."

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2.2.B—Describe the forces exerted on an object or system using a free-body diagram question 1