A 40 kg boy sits at 1.2 m from the fulcrum of the see-saw. Where should another boy of 30 kg sit on the other side of the see-saw in order to balance it? (2 marks)

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Solution

The boy sitting on the see-saw applies a moment of force on it due his own weight. To balance it, the boy of 30 kg weight will have to sit at that distance from the fulcrum which produces an equal moment of force.
Therefore,
Moment of force = Force $\times$ Perpendicualr distance

Since, both should produce the same moment of force

40 $\times$ 1.2 = 30 $\times$ d
d = 1.6 m

Therefore, the other boy should sit at a distance of 1.6 m from the fulcrum of the see saw in order to balance it. (2 marks)

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