Two forces, each of magnitude F, act along the edges of a rectangular metal plate, as shown.

Question 16

Two forces, each of magnitude F, act along the edges of a rectangular metal plate, as shown.

The plate has length a and width b.

What is the torque about point P?

A Fa                B Fb                C 2Fa              D 2Fb

Reference: Past Exam Paper – November 2017 Paper 13 Q12

Solution:

Answer: B.

Let the point P be at a height x about the surface.

In this case, consider P to be the pivot.

Torque (Moment) = Force × perpendicular distance of line of action of force to pivot.

Both forces F cause a clockwise moment about P.

The line of action of the force F on the right side of the plate is at a distance (b – x) above point P.

This force causes a torque = F × (b – x)

The line of action of the force F on the left side of the plate is at a distance x below point P.

This force causes a torque = F × x

Total torque about point P = F×(b – x) + (F × x)

Total torque about point P = Fb – Fx + Fx = Fb