Physics
Fluid-Mechanics
Easy

Question

A wooden cylinder of diameter 4r, height h and density rho over 3 is kept on a hole of diameter 2r of tank, filled with liquid of density  as shown in the figure.
Now level of the liquid starts decreasing slowly. When the level of liquid is at a height habove the cylinder the block starts moving up. At what value of h1 will the block rise?

  1. fraction numerator 4 h over denominator 9 end fraction
  2. fraction numerator 5 h over denominator 9 end fraction
  3. fraction numerator 5 h over denominator 3 end fraction
  4. Remains same

The correct answer is: fraction numerator 5 h over denominator 3 end fraction


    text  Let  end text A subscript 1 equals text  Area of cross-section of cylinder  end text equals 4 pi r squared
A subscript 2 equals text  Area of base of cylinder in air  end text equals pi squared
text  and  end text A subscript 3 equals text  Area of base of cylinder in water  end text equals A subscript 1 minus A subscript 2 equals 3 pi r squared
    Drawing free body diagram of cylinder
    text  Equating the net downward forces and net upward forces, we get,  end text h subscript 1 equals 5 over 3 h

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