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Question

# A 10 N metal block is suspended by a spring balance. A beaker containing some water is placed on a weighing machine which reads 20 N. The spring balance is now lowered so that the block gets immersed in water. The spring balance now reads 8 N. The reading of the weighing machine is

A

18 N
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B

24 N
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C

22 N
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D

28 N
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Solution

## The correct option is C 22 N Weight of the block, W1=10 N Weight of the beaker, W2=20 N Now, the spring balance is lowered and the block gets immersed in water. For spring and block system: T = Tension in the spring W1 = Weight of block U = Upthrust applied by water At equilibrium: T+U=W1⇒U=W1–T=10–8⇒U=2N…..(i) The block applies same force downwards. Now, for the system of water, beaker and weighing machine: Reaction force applied by weighing machine, R=U+W2⇒R=2+20⇒R=22N The weighing machine always measures the value of normal reaction R. Thus, the reading of the weighing machine is 22 N. Hence, the correct answer is option (c).

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