A pump motor is used to deliver water at a certain rate from a given pipe. To obtain, twice as much water from the same pipe, in the same time, the power of motor has to be increased to:

If the motor pumps water (density = ρ) continuously through a pipe of area of cross section A with velocity v, then mass flowing out per second, m = Avρ Rate of increase of kinetic energy = power of pump:P = 12mv2 = 12(Avρ)v2 = 12Aρv3To get double amount of water, velocity should be doubled. Hence power becomes 8 times.