GATE ME 2016 Set 2 — Question 51
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Fluid Mechanics → Fluid Properties & Statics → Forces on Submerged Bodies
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Question
Consider a frictionless, massless and leak-proof plug blocking a rectangular hole of dimensions at the bottom of an open tank as shown in the figure. The head of the plug has the shape of a semi-cylinder of radius . The tank is filled with a liquid of density up to the tip of the plug. The gravitational acceleration is . Neglect the effect of the atmospheric pressure.
The force required to hold the plug in its position is

Correct answer
(A) 2ρ R² gL (1 - (π)/(4))
Solution
The force required to hold the plug is equal to the net vertical hydrostatic force acting on the semi-cylindrical surface.
The vertical force on a curved surface is equal to the weight of the liquid (real or imaginary) supported by the curved surface up to the free surface.
In this case, the volume of liquid above the curved surface is the volume of a rectangular block of dimensions minus the volume of the semi-cylinder.
Volume of rectangular block,
Volume of semi-cylinder,
Net volume of liquid above the surface,
The vertical force is the weight of this volume of liquid:Therefore, option (A) is correct.
The vertical force on a curved surface is equal to the weight of the liquid (real or imaginary) supported by the curved surface up to the free surface.
In this case, the volume of liquid above the curved surface is the volume of a rectangular block of dimensions minus the volume of the semi-cylinder.
Volume of rectangular block,
Volume of semi-cylinder,
Net volume of liquid above the surface,
The vertical force is the weight of this volume of liquid:Therefore, option (A) is correct.
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