Question

A see-saw 8 m long is balanced in the middle. Two children of mass 30 kgf and 40 kgf are sitting on the same side of the fulcrum at a distance of 1.5 m and 3.5 m from the fulcrum respectively. Where must a lady weighing 60 kgf sit from the fulcrum, so as to balance the see-saw?

Answer 1

Let the lady sit at a distance d from the fulcrum on the opposite side to the children.

Clockwise moment = 30 kgf × 1.5 m + 40 kgf × 3.5 m

= 45 kgf m + 140 kgf m

= 185 kgf m

Anticlockwise moment = 60 kgf × d

∴ By the law of moments,

60 kgf × d = 185 kgf m

\[\therefore \text{d} = \frac{185}{60}\ \text{m} \]

= 3.08 m

The lady must sit 3.08 m from the fulcrum on the opposite side to the children.