Question

Two girls of masses 20 kg and 24kg
respectively sit on a see-saw 4.0 m
long balanced at the midpoint. The
20 kg girl is 1.8 m to the left of the
balance point. How far from the
balance point rest the other girl sit
to keep the see-saw horizontal.
(1.5m)​

Answer 1

Explanation:

Let the two children be sitting on the left arm. They will produce  anti-clockwise moment due to their weights about the middle point.

Total Anti-clockwise moment = 30 kgf x 2m + 50kgf x 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 clockwise moment). Let his distance from the middle be x m. Then

Clockwise moment = 74 kgf x X m = 74x kgf m 

By the principle of moments,

Anti-clockwise moment = Clockwise moment

185 = 74x

or x = 185/74m

= 2.5 m (on the other side).