How does torque work? Why can we assume the the net torque is equal to zero when we pick one of the points as the axis of rotation. For example if we selected the left leg of the table for the axis of rotation why can we assume the torque from the object plus the torque of the wood be equal in magnitude compared to the toque of the right leg?  
 A: The reason the net torque is assumed to be zero is because you are dealing with a system in equilibrium. A system in equilibrium should meet two essential conditions:


*

*The sum of the forces on the system equals to zero
$$\Sigma F=0$$

*The sum of the torques on the system must equal to zero
$$\Sigma τ = 0$$
A: You don't assume that the torque is equal to zero about any chosen point.  By observation, if the table is not rotating, it is not rotating about any point.  The observation is equivalent to a measurement, and that measurement tells you that the object is not rotating.
A: Torques are always calculated about some point. They are not independent of reference frames. Torques tell you have the angular momentum about a point is changing. 
If you observe (or are told) that an object has constant angular momentum (e.g., zero, not rotating, and staying not rotating) about any point in your reference frame, you can be certain that the torque about any point in your reference frame is zero.  Consequently, you are free to choose any point you wish for the torque calculation, and that result must give zero.  
