Why cannot a couple acting on a rigid body produce translatory motion

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, 

A uniform rod of length $l$ and mass $m$ is free to rotate in a vertical plane about $A$. The rod initially in horizontal position is released. The initial angular acceleration of the rod is (Moment of inertia of rod about $A$ is $\frac{m{l}^2}{3}$)

A mass $m$ is supported by a massless string wound around a uniform hollow cylinder of mass $m$ and radius $R$. If the string does not slip on the cylinder, then with what acceleration will the mass release? (Assume $g=$ acceleration due to gravity)

A rigid body of mass $M$ and radius $R$ rolls without slipping on an inclined plane of inclination $\theta ,$ under gravity. Match the type of body Column I with magnitude of the force of friction Column II.

A uniform metre rule placed on a fulcrum at its middle point $O$ and having a weight $40 gf$ at $10 cm$ mark and a weight of $20 gf$ at the $90 cm$ mark, Is the metre rule in equilibrium? If not how will the rule turn?