Case1:

The first condition for equilibrium does not ensure that a body is in equilibrium.

Consider a body pulled by the forces F1 and F2. The two forces are equal but opposite to each other. Both are acting along the same line; hence, their results will be zero. According to the first condition, the body will be in equilibrium.

Case2:

Now shift the location of the forces as shown in the figure. In this situation, the body is not in equilibrium although the first condition of equilibrium is still satisfied.it is because the body tends to rotate. This situation demands another condition for equilibrium in addition to the first condition i.e. second condition of equilibrium. According to this, a body satisfies the second condition when the resultant torque acting on it is zero.

∑  τ = 0