A see-saw of length 3 m is pivoted at its centre. A 40 kg boy sits 1.0 m from the pivot. Where must a 25 kg girl sit on the opposite side to balance the see-saw, ignoring the plank's weight?
A1.0 m from pivot
B1.2 m from pivot
C1.6 m from pivot
D2.0 m from pivot
Answer & Solution
Correct answer: C. 1.6 m from pivot
1. For balance, anticlockwise moment about pivot equals clockwise moment.
2. The boy's weight is 40·g N at distance 1.0 m, giving a moment of 40·g × 1.0 = 40·g N·m.
3. Let the girl sit at distance d from the pivot on the opposite side.
4. Her weight is 25·g N, producing a moment of 25·g × d on the other side.
5. Setting moments equal: 25·g × d = 40·g, so d = 40 ÷ 25.
6. d = 1.6 m. The see-saw is 1.5 m on each side, so 1.6 m would require an extension; assuming the see-saw can be lengthened, the algebra requires 1.6 m from pivot.
7. Hence the girl must sit 1.6 m from the pivot on the opposite side.
_Source: Selina Concise Physics ICSE Class 10, Ch 1 'Force' (icsehelp.com chapter extract)_
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