Question

On a see-saw, two children of masses 30 kg and 50 kg are sitting on one side of it at distance 2 m and 2.5 m respectively, from its middle, where should a man of mass 74 kg sit to balance it?

Answer 1

Let the two children be sitting on the left arm. They will produce an anti-clockwise moment due to their weights about the middle point.
Total Anti-clockwise moment = 30 kgf × 2 m + 50 kgf × 2.5 m
= 60 kgf m + 125 kgf m = 185 kgf m
To balance it, the man should sit on the right arm (to produce a clockwise moment). Let his distance from the middle be x m. Then

Clockwise moment = 74 kgf × x m = 74 x kgf m

By thr principle of moments,

Anti-clockwise moment = Clockwise moment

185 = 74 x

or x =`185/74` m

= 2.5 m (on the other side).