Define the moment of a force.
A thin disc of radius $r$ is held vertically by a pin at its centre $O$. Three forces act in the plane of the disc, as shown in Fig. 2.1. Two horizontal forces of equal magnitude $1.2\,\text{N}$ but opposite direction act at points $A$ and $B$ on the rim of the disc. A force of $6.0\,\text{N}$, acting at an angle $\theta$ below the horizontal, is applied at the midpoint $C$ of a radial line on the disc, as shown in Fig. 2.1. The disc’s weight is negligible and it is in equilibrium.
State an expression, in terms of $r$, for the torque of the couple due to the forces at $A$ and $B$ acting on the disc.
Friction between the disc and the pin is negligible. Determine the angle $\theta$.
State the magnitude of the force of the pin on the disc.