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In a book, Conceptual Physics by Benjamin Corral, it was given that when water falls down from a tap, it is a case of conservation of mass. They said that this is because water is thick when just comes out of the tap and then as it falls it becomes thinner and thinner.

The stream of water is fatter near the mouth of the faucet, and skinnier lower down. This can be understood using conservation of mass. Since water is being neither created nor destroyed, the mass of the water that leaves the faucet in one second must be the same as the amount that flows past a lower point in the same time interval. The water speeds up as it falls, so the two quantities of water can only be equal if the stream is narrower at the bottom.

How is it an example of conservation of mass?

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  • $\begingroup$ I find your question unclear. Are you asking 'Is mass conserved when it's made of water falling from a tap? Why? How?' or 'How is the thinning of the water flow related to the conservation of mass?' ? $\endgroup$
    – Steven Mathey
    Commented Feb 12, 2016 at 12:35
  • $\begingroup$ The book is Conceptual Physics by Benjamin Corral $\endgroup$
    – user101184
    Commented Feb 12, 2016 at 12:38
  • $\begingroup$ A stream of water example 1 The stream of water is fatter near the mouth of the faucet, and skinnier lower down. This can be understood using conservation of mass. Since water is being neither created nor destroyed, the mass of the water that leaves the faucet in one second must be the same as the amount that flows past a lower point in the same time interval. The water speeds up as it falls, so the two quantities of water can only be equal if the stream is narrower at the bottom. $\endgroup$
    – user101184
    Commented Feb 12, 2016 at 12:40
  • $\begingroup$ The book quotes this $\endgroup$
    – user101184
    Commented Feb 12, 2016 at 12:40
  • $\begingroup$ I wanted to know this more clearly. $\endgroup$
    – user101184
    Commented Feb 12, 2016 at 12:41

2 Answers 2

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It is actually an example of continuity equation, which might help you to understand the process. The continuity equation of course implies mass conservation, in a given case.

I find this example very useful as it contrasts typical consideration of continuity equation in which there is a flow of liquid through a pipe of variable diameter and speed of liquid is inversely proportional to the cross-section area.

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By "it is a case of conservation of mass. " Book means , that it uses Conservation of mass and is not an example of it.

Or more clearly,

when water falls down from a tap, it is a case which uses conservation of mass.

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