Faucet Physics Question

Bork · #1 03-13-2005, 09:29 PM

Why does a stream of water coming out of a faucet narrow as it falls?

Are there any equations which model the diameter of the stream as a function of the distance below the faucet?
The initial speed of the water and the diameter of the faucet are given constants.

Voltron87 · #2 03-13-2005, 09:33 PM

Nice first post.

I think I understand what you're saying, but I doubt there is an equation for that as there are way too many variables.

bholdr · #3 03-13-2005, 09:35 PM

simple answer: the stream narrows because of the combination of acceleration of the falling water and surface tension holding the stream together.

complicated answer: how the hell should i know? i'm sure there is an equation, but i'm just as sure i wouldn't understand it.

daryn may know, wacki could find it...

daryn · #4 03-13-2005, 09:44 PM

hmm i did take fluid mechanics. i know we talked about this too. i think it has to do with conservation of mass laws.

picture it like this, the water falls faster near the bottom of the stream than near the top. the same amount of mass has to pass through certain areas of the stream in the same amount of time, so it has to narrow as it goes faster.

MelchyBeau · #5 03-13-2005, 09:44 PM

water is attracted to itself. amazing I know. water is also attracted to edges. so there is going to be water falling from the edge of the faucet. but as it falls, it becomes attracted to the other water, thus it meets.

Melch (wacki isn't the only scientist on these forums)

edtost · #6 03-13-2005, 10:15 PM

so velocity is proportional to cross-sectional area?

wacki · #7 03-13-2005, 10:16 PM

[ QUOTE ]
(wacki isn't the only scientist on these forums)

[/ QUOTE ]

Cool, what specialty are you in?

Sweaburg · #8 03-13-2005, 10:29 PM

Inversley proportional...in a pipe at least.

Patrick del Poker Grande · #9 03-13-2005, 11:14 PM

[ QUOTE ]
the same amount of mass has to pass through certain areas of the stream in the same amount of time

[/ QUOTE ]
It's been quite a while since fluid mechanics for me, but I really doubt this. Why do you have to have conservation of mass through any particular area of the stream? There's nothing about this system that requires this.

Patrick del Poker Grande · #10 03-13-2005, 11:15 PM

[ QUOTE ]
Inversley proportional...in a pipe at least.

[/ QUOTE ]
For the same mass flow rate and running at or near capacity, yes, but not in general.