Question

A, B and C are the three forces each of magnitude 4 N acting in the plane of the paper as shown in Figure. The point O lies in the same plane.

1. Which force has the least moment about O? Give a reason.
2. Which force has the greatest moment about O? Give a reason.
3. Name the forces producing
   (a) Clockwise
   (b) anticlockwise moments.
4. What is the resultant torque about the point O?

Answer 1

i. C, as force C is the closest to O.

ii. A, because force A is farthest from O.

1. 1. A and B are producing clockwise moment.
   2. Moments are occurring anticlockwise due to the C force.

iii. The clockwise moment of force about O

= Force × OA

= 4N × 0.9 m

= 3.6 Nm

The clockwise moment of force about O

= Force × OB

= 4N × 0.8 m

= 3.2 Nm

Hence, the total clockwise moment of the two forces is about O = (3.6 + 3.2) Nm

= 6.8 Nm

Now, the anticlockwise moment of force about O = Force × OB

= 4N × 0.6 m

= 2.4 Nm

The resultant torque about the point O = clockwise moments − anticlockwise moments

= 6.8 Nm − 2.4 Nm

= 4.4 Nm (clockwise)