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Action and reaction, isn't it?
It is basically action and reaction just like a rocket the faster the mass goes out the back the more force there is.
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 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 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.
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
"Most of the force is produced because the water accelerates significantly in the tapered part of the nozzle."
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.