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## Homework Statement

A toy rocket consists of a plastic bottle which is partially filled with water. The space above the water contains compressed air. At one instant during the flight of the rocket, water of density p is forced through the nozzle of radius r at speed v relative to the nozzle. Determine, in terms of p, r and v, the mass of water ejected per unit time from the nozzle

## Homework Equations

## The Attempt at a Solution

I know the answer is πr

^{2}pv. But I am not sure of exact explanation why this is the case.