Question

Which one is not the condition for the equilibrium in free body diagram for calculation of the normal forces, consider all forces to be straight and linear?

(a) ∑Fx=0

(b) ∑Fy=0

(c) ∑Fz=0

(d) ∑F≠0

I had been asked this question in an interview.

I'd like to ask this question from Characteristics of Dry Friction topic in portion Friction of Engineering Mechanics

Answer 1

Right answer is (d) ∑F≠0

Easy explanation: For the equilibrium in the three dimensional system of axis we have all the conditions true as, ∑Fx=0, ∑Fy=0 and ∑Fz=0. Also, we have the summation of the forces equal to zero. Which is not a non-zero value.