Question

A seesaw is 3 metres long. It is balanced when child A sits 0.8 m from the pivot and child B sits 0.6 m from the pivot. Child A has a weight of 300 N. i. Draw a force diagram to represent this system. (Ensure that you label the force and distances given.) ii. Use the principle of moments to determine the weight of child B.

Answer 1

i) see attached diagram

ii) 400 N

Explanation:

Since we don’t know the mass of the seesaw, take moments about M (so we don’t have to consider R or mg).

Moment = Force x Perpendicular Distance to the Pivot

As the system is in equilibrium, total anti-clockwise moment = total clockwise moment:

⇒ 300 × 0.8 = b × 0.6

⇒ 0.6 b = 240

⇒ b = 400 N