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Name: Brian
Status: Student
Grade: Other
Location: NC
Country: United States
Date: May 2007


Question:
We have a little bet here if you put your finger over the end of a water hose is the pressure the same, less, or greater than when your finger is not over the end of the hose?



Replies:
The problem with betting on science is that you have to define the bet very carefully... :)

The answer depends on where you are measuring the pressure... just before your thumb, the pressure is higher (closer to the upstream pressure). Just after the thumb, the pressure becomes the same (roughly atmospheric). This assumes the hose is horizontal (neglecting static pressure head).

Hope you won the bet!

Burr Zimmerman


Hi Brian,

It depends on where you are measuring the pressure, but the general answer is the pressure will rise when you block the end of the hose.

Let us look at a typical situation consisting of the water pipes in your home, connected to a tap, which then feeds a length of hose, that is free to discharge water from its open end. A typical water pressure in the pipes in your home is about 70 PSI (pounds per square inch). As the water flows through the tap, it encounters a restriction, so the result is that after the tap, the water pressure is much lower as it enters the hose. The remaining pressure pushes the water along to the overcome the resistance of the hose. As the water gets closer to the end of the hose, its pressure gets less and less, because there is less and less remaining hose that restricts the water's flow. Finally at the outlet of the hose, the water pressure falls to zero. The water may flow out strongly, but this is caused by its momentum, not pressure. It has been flowing fast inside the hose, and as it emerges, it naturally continues to flow just as quickly away from the hose, as it flowed inside the hose.

Now, if you block the end of the hose, all water flow stops. The restrictions to water flow (such as the resistance of the hose, and the restriction of the tap) do not matter any more, because there is no water flowing any more. Restrictions to water flow can only operate if water is flowing; but you have your finger over the end, so no water can flow. The pressure in all parts of the hose now rises to the same pressure as is in the house's pipes. Without your finger blocking the hose, the pressure at the end as the water flows out, is zero. When you block the end, you can certainly feel the pressure is no longer zero! So clearly, the pressure at the end of the hose rises very substantially when you block it.

So, Brian, who wins the bet?

Regards,

Bob Wilson.


Dear Brian,

If the end of the hose is open, the pressure will be just the pressure of the air -- atmospheric pressure. If you put your finger over the end of the hose so no water can escape, the pressure will increase to the pressure in the pipe when the faucet is closed. If the finger pressure is reduced so some water escapes, the pressure at the end of the hose will be intermediate.

Best, Dick Plano



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