In this model the pin that prevents the disk from falling under the influence of gravity, is located to the right of the center of mass. The light gray lines show the line of action of the weight vector acting at the center of mass and the distance from that line of action to the pin. That distance is called the moment arm.

If the disk were free to rotate about the axis of the pin, the torque about the axis of the pin due to the weight of the disk would produce an angular acceleration, causing the disk to swing initially to the right. As the disk swings, the line of action of the weight vector crosses to the other side of the line of action of the force applied by the pin, reversing the torque so that the disk swings back and forth like a pendulum.

For this apparatus to be in static equilibrium, the friction at the pin must supply a torque, without breaking away, sufficient to counter the weight induced torque. If the net torque is ever other than zero, static equilibrium is broken. Perhaps it would be a good idea to weld the disk to the pin. You may click on the Action button to reduce the coefficient of friction between the pin and disk to zero. Since you have completed the Rotational Dynamics section, you should be able toe explain the wild gyrations in the reaction force vector.

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