A see-saw is in equilibrium with a 30 kg child at 1.5 m from the pivot. Where should a 45 kg child sit so the see-saw is balanced?
A1.5 m from pivot on the opposite side
B0.75 m from pivot on the opposite side
C1.0 m from pivot on the opposite side
D2.0 m from pivot on the opposite side
Answer & Solution
Correct answer: C. 1.0 m from pivot on the opposite side
1. For equilibrium of the see-saw, anticlockwise moment equals clockwise moment about the pivot.
2. The 30 kg child has weight 30·g N at distance 1.5 m.
3. Anticlockwise moment = 30·g × 1.5 = 45·g N·m.
4. Let the 45 kg child sit at distance d m on the opposite side.
5. Clockwise moment = 45·g × d. Setting it equal to 45·g gives d = 1.0 m.
6. So the heavier child sits 1.0 m from the pivot.
_Source: Selina Concise Physics ICSE Class 10, Ch 1 'Force' (icsehelp.com chapter extract)_
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