A motor is used to deliver water through a pipe. Let the motor have a power P initially. To double the rate of flow of water through the pipe, the power is increased to P’. What is the value of P’/P?

(a) 2

(b) 4

(c) 6

(d) 8

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The correct answer is (d) 8

Easiest explanation: Power = Force x Velocity

= (Mass/Time) x Velocity^2

= Rate of flow x Velocity^2

Rate of flow = Area x Velocity

Doubling the rate of flow will double the velocity.

P = Rate of flow x Velocity^2

P’ = (2 x Rate of flow) x (2 x Velocity) ^2

= 8P

P/P’ = 8.