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# A cylinder containing water up to a height of 25 cm has a hole of cross-section in its bottom. It is counterpoised in a balance. What is the initial change in the balancing weight when water begins to flow out

- Increase of 12.5 gm-wt
- Increase of 6.25 gm-wt
- Decrease of 12.5 gm-wt
- Decrease of 6.25 gm-wt

## The correct answer is: Decrease of 12.5 gm-wt

### Let A = The area of cross section of the hole

v = Initial velocity of efflux

d = Density of water,

Initial volume of water flowing out per second = Av

Initial mass of water flowing out per second = Avd

Rate of change of momentum = Adv^{2}

Initial downward force on the flowing out water = Adv^{2}

So equal amount of reaction acts upwards on the cylinder.

\ Initial upward reaction = [As ]

Initial decrease in weight

gm-wt.

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Let A = cross-section of tank

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