Home > Technical > Did you Noticed – flow of a stream of water

Did you Noticed – flow of a stream of water

Q: If you watch water flowing out of the end of a hose, you’ll see that the stream of water is fatter near the mouth of the hose, and skinnier lower down. Why?

A: If you watch water flowing out of the end of a hose, you’ll see that the stream of water is fatter near the mouth of the hose, and skinnier lower down. This is because the water speeds up as it falls. If the cross-sectional area of the stream was equal all along its length, then the rate of flow (kilograms per second) through a lower cross-section would be greater than the rate of flow through a cross-section higher up. Since the flow is steady, the amount of water between the two cross-sections stays constant. Conservation of mass therefore requires that the cross-sectional area of the stream shrink in inverse proportion to the increasing speed of the falling water.

self-check: Suppose the you point the hose straight up, so that the water is rising rather than falling. What happens as the velocity gets smaller? What happens when the velocity becomes zero?

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