[PAOLO137 (OP)]

When trying to estinguish a fire, imagine two firemen pushing hard the metal end piece of the hose they are using to
send water on the flames. If there were other two firemen pulling the hose in the opposite direction, everithing will look logical. But these latter firemen are not there, nor any external object is in contact with the outer surface of the hose.
It is evident that the backward force originates on the internal surface. Must we conclude that a fluid pushed by some kind of pressure in a pipe exerts a force in the direction opposite to the flow of liquid?

[yor_on]

Action and reaction, isn't it?

A moving fluid has a net force pointing in the direction of its movement, as differing it from a same 'force/pressure/action' acting on a fluid from all sides, not moving and so being 'at rest'. So in a way it's a 'pressure imbalance' producing a 'action' in some direction that then is balanced by its reaction, eh, sort of

Newton. (in ppt format)

[Geezer]

Action and reaction, isn't it?

Yes, although I think it's more about F=ma.
 
Most of the force is produced because the water accelerates significantly in the tapered part of the nozzle. The force is determined by the mass of water flowing through the nozzle and the amount the water is accelerated.
 
If you removed the nozzle from the hose, the force on the firemen would be greatly reduced, and the water would not travel very far because it was not being accelerated. You can try this with a garden hose and you'll see what I mean.

[Soul Surfer]

It is basically action and reaction just like a rocket the faster the mass goes out the back the more force there is.  If you have ever used a pressure washer with an adjustable nozzle you will notice it.  When the nozzle is open water squirts gently out.  As you close it down the flow rate ges less but the pressure and velocity increases and the reaction force becomes quite large

[Geezer]

It is basically action and reaction just like a rocket the faster the mass goes out the back the more force there is.

It is not. Rockets work because they accelerate mass. Most of the reactive force is caused by the product of the mass times its acceleration. It's a question of how much the water is accelerated at the nozzle, not the exit velocity.

If there was no change in velocity over the length of the hose, the fireman would hardly have to resist any force.

As I said, you can easily prove this with a garden hose. Put a nozzle on the hose and adjust it for maximum reaction. Now cut the hose and throw away the nozzle. The reaction is negligible because the acceleration is negligible, but the mass flow has increased.

Now do it the other way. Adjust the nozzle for maximum distance (therefore maximum acceleration) and you'll find that the reaction is negligible because the mass flow is negligible.

It's not rocket science! (err, well I suppose it is)

[Soul Surfer]

we may be talking at cross purposes here Geezer the main reaction is produced by the kinetic energy of the water leaving the end of the hose  This is 1/2 m.v^2  the mass flow may be reduced but the increase in velocity can overcome this.  What you suggest is that if put an open ended flexible hose pipe on the ground and run water through it quickly so there is no net acceleration at the end it will lie there and do nothing.  Try this experiment but wear waterproof clothing.   

[Geezer]

we may be talking at cross purposes here Geezer the main reaction is produced by the kinetic energy of the water leaving the end of the hose  This is 1/2 m.v^2  the mass flow may be reduced but the increase in velocity can overcome this.  What you suggest is that if put an open ended flexible hose pipe on the ground and run water through it quickly so there is no net acceleration at the end it will lie there and do nothing.  Try this experiment but wear waterproof clothing.   

I don't think we are talking at cross purposes.

The water is significantly accelerated at the nozzle. Unless you are saying the nozzle does not accelerate the water, you have to account for the reaction to that acceleration. The firemen provide the reaction.

[Soul Surfer]

I am talking about a longish open ended hosepipe of constant diameter with no nozzle. The water comes out as a clean solid straight jet of constant width equal to that of the internal diameter of the hosepipe for quite a considerable distance. There can therefore be no acceleration in flow rate near the jet.  If it is coming out quite fast you will feel the reaction force and if it is just lying on the ground the reaction will cause the hosepipe to thrash around in a very interesting manner.

Maybe we should try an experiment?

[Soul Surfer]

I tried to find something on youtube but could not and I am beginning to have second thoughts that you may have to have acceleration.  However I am sure That I have managed to get an open ended hose pipe thrashing around with a very fast water flow in the past.  Mind you it may have been the curve in the pipe that did it.  Interesting.

[Geezer]

This was my reply to your previous post.

I am talking about a longish open ended hosepipe of constant diameter with no nozzle. The water comes out as a clean solid straight jet of constant width equal to that of the internal diameter of the hosepipe for quite a considerable distance. There can therefore be no acceleration in flow rate near the jet.  If it is coming out quite fast you will feel the reaction force and if it is just lying on the ground the reaction will cause the hosepipe to thrash around in a very interesting manner.

I agree with that, but it's nothing like how a fire-hose actually works! (Which was the OP's question)

The water in the hose enters the nozzle at a (lowish) velocity and exits the nozzle at a much greater velocity. A considerable mass of water was accelerated (work done by the pump) in the nozzle. Unless the hose is a rigid pipe that goes all the way back to the pump, it's simply impossible to ignore the reaction produced by this acceleration. Something has to provide the reactive force, and the only thing around that can is the firefighters.

If you run the numbers I think you will discover that is where most of the force exerted on the firefighters comes from. I say "most", because the water does have some kinetic energy as it enters the nozzle, but it's the change in kinetic energy across the nozzle that produces most of the reactive force. Obviously it depends on the relative cross sectional areas of the hose and the nozzle exit, but the whole idea of fire-hoses is to deliver the maximum mass flow to the nozzle with minimum pressure drop, and that means keeping the velocity in the hose as low as possible.

[Geezer]

I tried to find something on youtube but could not and I am beginning to have second thoughts that you may have to have acceleration.  However I am sure That I have managed to get an open ended hose pipe thrashing around with a very fast water flow in the past.  Mind you it may have been the curve in the pipe that did it.  Interesting.

Agreed - it will thrash around if there is sufficient pressure, so it is producing some thrust, but when you add a nozzle you can increase the thrust dramatically, even though you have reduced the mass flow.

[Soul Surfer]

Yes I agree that having the flow restricted and the pressure used to accelerate the water will produce more thrust but any flow produces some reaction just looking at the acceleration that takes place will not give an accurate answer because it does not include the bit involved in the basic flow rate.  If you use the kinetic energy output it should give an accurate output because this effectively includes both parts of the answer.  A lot of calculations of this type are best handled using an energy approach.

[Geezer]

Yes I agree that having the flow restricted and the pressure used to accelerate the water will produce more thrust but any flow produces some reaction

Yes - that's why I originally said;

"Most of the force is produced because the water accelerates significantly in the tapered part of the nozzle."

[Geezer]

"Most of the force is produced because the water accelerates significantly in the tapered part of the nozzle."

I'll have to take that back.

I should have said;

"ALL of the force is produced because the water accelerates significantly in the tapered part of the nozzle."

Thrust is only produced when (and where) a fluid is accelerated. Water running out of a constant diameter pipe or hose is not producing any thrust at the end of the pipe or hose. The water may have kinetic energy, but its velocity did not change, so there can be no thrust. A hose may flop around, but that is because the hose is altering the direction of the mass flow, not because of thrust.

It is not possible to determine the reaction force supported by the firemen if the only data available is the mass rate and velocity of the water leaving the end of the hose. It could be anywhere between zero (if all the acceleration takes place at the pump) and a large number (if all the acceleration takes place at the nozzle).

Typically, all the acceleration happens at the nozzle, but the thrust (and therefore the reaction on the firefighters) depends entirely on the change in velocity of the water across the nozzle.

I did a bit of poking around, and there is a lot of discussion in the context of jets about Newton's third law, but there is hardly any discussion at all about that which produces the force that needs to be reacted to (Newton's second law).

There ain'ta a no sanity clause, and if nothing gets accelerated, there ain'ta no force! (and you can quote me on that.)

[yor_on]

If we had a frictionless material for the 'fire-hose' of a same diameter, no bends and no nozzle, then there would be no 'force'? It's a nice way to think of it actually and it could be applied to a lot of things inside SpaceTime if correct.

[PAOLO137 (OP)]

First of all I wish to thank all the friends who took some of their time to answer my question. Now I can tell all the story about it. In 1958 as a student undergoing my so called "Physics 1" examination on the first year at University of Rome, the professor asked me about why the firemen were pushing the hose. I remember well having taught that answering it as an occurrence of the third principle of dynamics would have been too a stupid and simple question for a university student in physics! So I concluded the professor was asking for something more deep. As an example, I suppose all of you know the arrangement of a gun fixed over a carriage, which,when the gun is fired, starts  running backwords while the ball goes forward so that MV = - mv which is simply the conservation of momentum. But someone likes to call it "the third principle". So i stood silent trying to consider all the forces in play, while the professor shouted "have you ever heard about the third principle of dynamic?". I felt ashamed but he was really a stupid person. Anyway the problem of explaining the phenomenon starting from first principles has always puzzled me. I see that many of you refer to the form of the nozzle, but those of you who are experts of history of cinema will remember the first movie by the Lumiéres brothers "L'arroseur arrosè" where there is no nozzle as every gardner knows. Thanks to everybody anyway. I have still second thoughts about it.

[Geezer]

If we had a frictionless material for the 'fire-hose' of a same diameter, no bends and no nozzle, then there would be no 'force'? It's a nice way to think of it actually and it could be applied to a lot of things inside SpaceTime if correct.

I guess so. I was thinking along the same lines. If you put the nozzle all the way back at the pump you could accelerate the flow at that point and conduct the accelerated water through a hose to the point of discharge.
 
In that case, there would be no reaction on the firefighters at all but, as you point out, it could only work with a perfectly straight, frictionless, hose.

[JP]

Conservation of momentum would tell you the hose wouldn't move at all unless the water accelerates. 

The water going in has momentum = mv.  The water coming out at the same velocity has momentum = mv.  Since input momentum = output momentum, the hose can't pick up momentum. 

[Soul Surfer]

OK you have convinced me Geezer, there would be no forces as long as the hose is dead straight. any bend  (and there almost always is) and it will start to thrash around and the thrashing will produce more bending and so on. It is an unstable and dynamic situation.  Even more interesting.

The acceleration forces will be produced further back probably at the tap.  When you come to think of it the same is true of a liquid fuelled rocket motor the fuel flows relatively slowly into the combustion chamber and the gasses are accelerated out the back by the combustion.

It just goes to show that a lot of these fluid dynamics situations are far from obvious.  I have just had to revise my mental model of what can happen as material collapses under gravity when there is turbulent angular momentum in the material. one tends to think of it collapsing towards the centre and just speeding up towards the cental blob and or disc.  However recent work shows there is quite a lot of "water down the plughole" effects of local vortices forming and the material going "stringy" as it collapses.

[Geezer]

It certainly had me fooled too SS!

Poking around I found that "nozzle reaction" is well known to firefighters, and there are a lot of explanations about what's going on, but all the ones I saw just say it's a reaction to the water coming out the hose. There seems to be a common misconception that it requires a reactive force. The reaction is to the force produced by accelerating mass, but none of the descriptions seem to mention that.

Same with "bottle rockets". Again is supposed to be a reaction to the water coming out of the nozzle, but that's not right either. And, as you say, rockets in general are sometimes misunderstood too.

Newton would not be a happy camper!