A see-saw will balance when the pivot is placed at the center of mass of the system.If a boy and his brother sit at distances 3L and L from the pivot,respectively,and the see-saw is balanced,what is the boy's mass if his brother's is 85 kg? Assume the see-saw board is uniform.
A)28 kg
B)64 kg
C)21 kg
D)57 kg

Question

Quizplus · Accepted Answer

Since the see-saw is balanced, the torque on either side must be equal. The torque on each side is given by the product of the weight and the distance from the pivot. Let M be the mass of the boy sitting at a distance 3L from the pivot. Then, we have:

M * g * 3L = 85 kg * g * L

where g is the acceleration due to gravity. Solving for M, we get:

M = (85 kg * L) / (3L) = 28.33 kg ≈ 28 kg

Therefore, the boy's mass is approximately 28 kg. Answer choice A) is the best choice.

A See-Saw Will Balance When the Pivot Is Placed at the Center