Q#78: [Forces > Moments]
Rigid circular disc of radius r has its centre at X. A number of forces of equal magnitude F act at edge of disc. All the forces are in plane of the disc.
Which arrangement of forces provides moment of magnitude 2Fr about X?
Reference: Past Exam Paper – November 2006 Paper 1 Q14
Solution 78:
Answer: B.
Moment = Force (F) x perpendicular distance (r) of force from pivot X
In this question, moment produced by each force F is Fr
Moments can be either clockwise or anticlockwise.
Ans A: an anticlockwise moment of Fr is produced
Ans B: a total clockwise moment of (Fr + Fr =) 2Fr is produced (this is a couple – the forces (same magnitude) are in opposite direction, but act at different points so that they produce a turning effect - each force produces a moment which is clockwise. The resultant force is however zero.)
Ans C: net moment is zero (since clockwise and anticlockwise moments are both Fr. Resultant force is 2F here, not zero.)
Ans D: a net clockwise moment of (Fr + Fr – Fr = ) Fr is produced
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