Question

Which of the following is correct for the moment of couple M caused by two forces of same magnitude F and are separated by a distance of r?

(a) M = rxF

(b) M = r.F

(c) M = -(rxF)

(d) M = -(r.F)

I had been asked this question at a job interview.

My question is taken from Moment of a Couple topic in section Force System Resultants of Engineering Mechanics

Answer 1

Right choice is (a) M = rxF

The explanation: The answer is M = rxF. The moment id the cross product of the distance and the forces. It is the direction as directed by the curling of the hand. The thumb representing the direction of the moment. And the motion of the finger, that is the rotation gives the direction of the rotation caused by the forces.