Science Questions

Where does water in a hose pipe go?

Sun, 5th Jul 2009

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Question

Chris in Cavendish asked:

When youíve got 15 meters worth of a hose pipe for example and you switch off the water, it immediately stops coming out off the end so where does that 15 meters of water to go?

Answer

Ben -   It stays in the hose pipe, surely.

Chris -   Absolutely!  I mean thatíd be my thought, but thereís a couple of reasons why.  One is that fluids, liquids are incompressible and as a result, when itís a bit like a Newtonís cradle, you put some water at one end of the hose and it pushes an equivalent volume of water out the other end of the hose.  You could argue that water running through the hose has a bit of momentum but the point is that if youíve got a perfectly sealed hose and itís at the same height as the tube at the other end, so the gravity is not helping and you donít wait for an air bubble to go in and displace water out, the only way more water could come out is by creating a vacuum between the end of the tap in the tube.

Kat -   And that ainít going to happen, in nature...

Chris -   And thatís not going to happen quickly and so for that reason, the water turns off because youíre pushing something which is effectively incompressible out the end of tubes.  Thereís no effective stored elastic energy in there.

Kat -   So, if you got like a skewer or something and stuck it in the tap end of your hose, then the water will dribble out because youíre letting air in the hose.

Chris -   It would possibly come out quicker.  Itís a bit like shot gunning a can of drink, isnít it?  When you turn a can of drink upside down, it goes glog, glog, glog, glog, glog because you have to replace the drink volume thatís coming out with an equivalent volume of air.  Otherwise, thereíll be a vacuum above the drink whereas if you get the can and make a hole in the bottom and then pop the top off, it will empty very, very fast because air can enter easily without having to form bubbles and pass through the liquid to let it go through.

Kat -   Shotgun your hose pipe.

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