Physics
Fluid-Mechanics
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Question

A  cylinder containing water up to a height of h cm has a hole of cross-section area a cm2 at its bottom. It is counterpoised in a balance. When water just begins to flow out, the initial change in weight is

  1. 2 text  gha  end text
  2. square root of 2 g h a end root
  3. square root of g h a end root
  4. None

The correct answer is: 2 text  gha  end text


    Let y be the initial velocity of efflux of water.
    Initial volume of water flowing out per sec = av
    Initial mass of water flowing out per sec equals a v d equals a v cross times 1 equals a v
    Rate of change of momentum equals left parenthesis a v right parenthesis v equals a v squared
    Initial downward force on the outflowing water =av2
    Reaction acts upwards on the cylinder
    Initial upward reaction = av2
    Now v equals square root of 2 g h end root
    Initial decrease in weight equals a cross times 2 g h equals 2 g h a

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