A cylinder containing water up to a height of 25 cm has a hole of cross-section 1/4 cm2  in its bottom. It is counterpoised in a balance. What is the initial change in the balancing weight when water begins to flow out?

Let A  be the area of cross-section of the hole, v  the initial velocity of efflux and d  the 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=(Avd)v=Adv2 Initial downward force on the out flowing water=Adv2 Reaction acts upwards on the cylinderInitial upward reaction=Adv2 Now, v=2gh ∴  Initial decrease in weight=(Ad)(2gh)=2Adgh=2(14)(1)(980)(25) =12.5×980 dyne=12.5 gm wt