[Geezer]

A slightly more graphic demonstration of the effect.
 
http://www.youtube.com/watch?v=QK_STipGPwk&feature=related

[Soul Surfer]

That's fantastic!  What I was looking for is video of a straight hose pipe without a nozzle thrashing around its really quite wild. It is now more understandable if it is the curvature that is causing the force rather than simple thrust from the end.

[yor_on]

Well Paolo, that professor sounds seriously uncool
But we seem to have reached a concord here at last.

[ka9q]

I found this while specifically looking for discussions about firehose reaction forces as a way to explain the rocket principle.

The water does not have to speed up in the nozzle. The hose can be completely smooth and cylindrical all the way from the pump to the nozzle and it will still produce a reaction force equal to the mass flow rate times the velocity at which the water comes out.

This is exactly the ideal rocket equation: thrust = mass rate * exhaust velocity. I found these tables of hose reaction forces

newbielink:http://www.firetactics.com/NOZZLE-REACTION.htm [nonactive]

independently calculated the mass flow rate and the stream velocity, and when I multiplied them I got exactly the values given in the table.

Where does this reaction force actually come from? From unbalanced water pressures on the inside of the hose. First consider a bent hose. Although the water does not speed up (because the diameter is fixed) it is nonetheless being accelerated around the bend just as someone on a merry-go-round is continually being accelerated toward the center creating what is commonly referred to as centrifugal force.

This acceleration requires the hose to exert a force on the water, and by Newton's 3rd law the water pushes back on the hose. The hose is flexible, so this can have a dramatic effect.

Normally, a pressurized fluid inside a closed container pushes equally in all directions so there's no net force. But the water can't push on the nozzle end of the hose because there's nothing there to push on -- it's free to come out. So the force of the water on the inside of the bend is unopposed, and the hose kicks back.

In the special case of the hose being absolutely straight, there will be no net force on the nozzle; all the force will be on the bend inside the pump just before the water comes out. But the slightest bend in the hose, or the slightest deflection of the water at the nozzle, will create a lateral force that will (if they're free to move) cause the hose to bend and the nozzle to move, increasing the bend and increasing the force and so on until it's whipping around quite impressively.

[wolfekeeper]

Yes, Geezer et al have it basically right, at least for a straight pipe, with bends in it and no nozzle.

"Tackling a loose hose"

"loose fire hose"

But if you have a narrowing nozzle on the end, and there's real-world water flowing, then there's a second, more subtle effect: Bernouilli effect. What happens is that the flow of water reduces the pressure right inside the nozzle around the opening, and that gives a net backwards force on the whole system (including the pump), even with a straight hose.

This is important in water rockets; it doubles the force due to the nozzle, if you just use the area of the nozzle exit and multiply by the pressure you get the wrong answer, by a factor of two.

[PAOLO137 (OP)]

thanks to everybody. At the end of the day I got a confirmation that the problem is a thorny one. Paolo, se you at the next question.