Question

Which of the following needs to zero for the perfect equilibrium?

(a) ∑F=0, ∑M=0 and ∑θ = 0

(b) ∑F=0, ∑M≠0 and ∑θ = 0

(c) ∑F≠0, ∑M=0 and ∑θ = 0

(d) ∑F=0, ∑M=0 and ∑θ≠0

I have been asked this question by my college professor while I was bunking the class.

This intriguing question originated from Conditions for a Rigid-Body Equilibrium topic in chapter Equilibrium of a Rigid Body of Engineering Mechanics

Answer 1

Right choice is (d) ∑F=0, ∑M=0 and ∑θ≠0

For explanation I would say: The summation of the forces needs to be zero. So does the summation of the moments need to zero. But talking about the angles, they not needed to zero. But the forces which are acting at particular angles, must need to be equal to zero. The basic need of the forces to make the body in equilibrium.