Question

If a rigid body acted upon by a system of coplanar parallel forces is to be in equilibrium, then :

• A. Vector sum of the forces must be zero
• B. Vector sum of the moments of the forces taken about a point in their plane must be zero
• C. One of the conditions (A) or (B) must be satisfied
• D. Both the conditions (A) and (B) must be satisfied

Answer 1

The correct option is D Both the conditions (A) and (B) must be satisfied

For equilibrium, we have

By Newton's 2nd law

$F_{net}=0$

And by conservation of torque, we have

${\tau }_{net}=0$

Hence, both conditions A and B must be satisfied. Hence option D.