Two parallel forces, each of magnitude F, act on a rod of length 5d.

Question 37

Two parallel forces, each of magnitude F, act on a rod of length 5d.

Which diagram shows the positions of the two forces that will produce the largest torque on the rod?

Reference: Past Exam Paper – June 2018 Paper 11 Q12

Solution:

Answer: D.

Torque is the turning effect caused by the forces on the rod. The forces that will produce the largest torque on the rod are the forces that cause the rod to turn (rotate) by the greatest amount.

The forces in option B and C would causes the rod to move upwards. The rod would move upwards (this is a linear motion) instead of turning. [B and C incorrect]

The forces in A and D constitute a couple in each case. This causes the rod to rotate in one direction (clockwise or anticlockwise).

Torque of couple = one force × perpendicular distance between the 2 forces

In option A: Torque of couple = F × d = Fd

In option D: Torque of couple = F × 2d = 2Fd

 

So, the forces in D cause the largest torque.