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Question

The tension in a string holding a solid block (ρb=700 kg/m3) below the surface of a liquid (ρl=1000 kg/m3) as shown in figure is 10 N when the system is at rest. What will be the tension in the string if the system has an upward acceleration of 5 m/s2?
[Take g=10 m/s2]


A
10 N
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B
20 N
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C
15 N
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D
5 N
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Solution

The correct option is C 15 N
Let m be the mass of block.
Initially for the equilibrium of block,
F=T0+mg


Here, F is the upthrust on the block.
When the lift is accelerated upwards,
geff=g+a
Hence, F=Fgeffg


Form Newton's 2nd law
F(T+mg)=ma
Fgeffg(T+mg)=ma
(T0+mg)geffg=T+mg+ma
T0geffg+mgeff=T+m(g+a)
T=T0geffg
[geff=g+a]
T=10×(10+5)10=15 N
So, T=15 N will be the tension in the string.

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