Q#16 (Past Exam Paper – November 2017 Paper 13 Q12)
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
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
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