Second Condition of Equilibrium
An object in equilibrium does not move along a straight line -- it does not translate -- that means the sum of all the forces on it is zero. That was the first condition of equilibrium.
But an object in equilibrium also does not rotate. That means the sum of all the "rotational forces" or torques on it is also zero. The sum of all the torques on an object is equilibrium is zero. This is the Second Condition of Equilibrium.
Torques that would rotate an object counter clockwise may be taken as positive and torques that would rotate an object clockwise may be taken as negative. Then we can write this Second Condition of Equilibrium as
or we can calculate the sum of the clockwise torques and set them equal to the sum of the counterclockwise torques. Then we can write this Second Condition of Equilibrium as
Be sure all the torques are calculated about the same origin!
Torques Deformation of Solids Return ToC, Static Equilibrium (c) Doug Davis, 2001; all rights reserved