Water is flowing at a speed of 1.5 m/s through a horizontal tube of cross-sectional area 10 m². You are trying to stop the flow by your palm. Assuming that the water stops immediately after hitting the palm, what is the minimum force that you must exert? (density of water = 10³ kg/m³)

Answer is B
Force exerted will be due to the rate of change of momentum. Final momentum after hitting is zero. Therefore,
Let v be the velocity of water = 1.5 m/s
Let A be the area of cross section = 10 m²
Let ρ be the density of water = 10³ kg/m³
Mass flow rate = ρAv = 10³ kg/m³ × 10 m² × 1.5 m/s = 15000 kg/s
Change in momentum per second = initial momentum – final momentum = (15000 kg/s × 1.5 m/s) – 0 = 22500 kg m/s²
Force = rate of change of momentum = 22500 N = 22.5 kN
Therefore, the minimum force required is 22.5 N