The stream of water from a faucet decreases in diameter as it falls. Derive an equation for the diameter of the stream as a function of the distance y below the faucet, given that the water has speed v0 when it leaves the faucet, whose diameter is D. The water molecules in contact with the air are being slowed down because they are colliding with the air molecules (essentially friction). These water molecules in turn slow down other water molecules in nearby water layers and so on. This is much is clear. However, I don't know how to model this situation mathematically. Can anybody give me a start-push?