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Spring balance A reads 2 kg with a block of mass m hanging from it. Spring balance B reads 5 kg when a beaker containing liquid rests on its pan. The setup is then rearranged so that the hanging block is fully submerged in the liquid (without touching the beaker). In this new situation:

1. balance A will read more than 2 kg.
2. balance B will read more than 5 kg.
3. balance A will read less than 2 kg and B will read less than 5 kg.
4. balances A and B will read 2 kg and 5 kg respectively.

Correct answer: balance B will read more than 5 kg.

Solution

Buoyancy reduces the tension in A, so A reads less than 2 kg. The reaction force pushes down on the liquid/beaker, so B reads more than 5 kg. Among the given options, the correct true statement is that B reads more than 5 kg.

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