Moment of a couple can be defined as the tendency of the force to rotate.

A wheel of radius, $R$ with an axle of radius, $\frac{R}{2}$ is as shown in the figure and is free to rotate about a frictionless axis through its center and perpendicular to the page. Three forces $\left(F, F, 2 F\right)$, are exerted tangentially to the respective rims as shown in the figure.

The magnitude of the net torque acting on the system is nearly, 

One end of a thin bar $AB$ of mass $m$ and length $l$ is hanging from a massless string. The other end is held at an angle of $45°$ with horizontal. Find the tension in the string, immediately after the release of the bar.