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Question

When a boy weighing 20 kg sits at one end of a 4 m long see-saw, it gets depressed at this end. How can it be brought to the horizontal position by another boy weighing 40 kg?


A

By sitting at a distance of 1 m from the centre on the side opposite to the first boy.

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B

By sitting at a distance of 1 m from the centre on the same side as the first boy.

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C

By sitting at a distance of 2 m from the centre on the side opposite to the first boy.

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D

By sitting at a distance of 2 mfrom the centre on the same side as the first boy.

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Solution

The correct option is A

By sitting at a distance of 1 m from the centre on the side opposite to the first boy.


Weights of the two boys : W1 = 20 kg, W2 ​ = 40 kg

Length of see-saw = 4 m
Distance from the centre = 2 m
Moment of Force = Force × perpendicular distance
Then, the principle of moments can be written as follows:
Moment in clockwise direction = Moment in anticlockwise direction
W1× d1​ = W2× d2

20 × 2 = 40 × d2​​

d2​​​ = 20×240 = 1 m

Thus, the other boy can sit at a distance of 1m from the centre on the side opposite to the first boy, so that the long see - saw can be brought to the horizontal position.


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