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An engine drives water through a hose. The water emerges from the hose at a speed of 2 m/s. If the mass of water contained per unit length of the hose is 100 kg/m, what power does the engine deliver?

1. 400 W
2. 200 W
3. 100 W
4. 800 W

Correct answer: 400 W

Solution

Mass flow rate = (mass per length)(speed) = 100*2 = 200 kg/s. Power delivered = rate of kinetic energy = (1/2)(dm/dt)v^2 = (1/2)(200)(2^2) = 400 W.

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