I Nozzle Reaction Forces: Explaining the Resultant Force

AI Thread Summary
The discussion centers on the reaction forces experienced by nozzles in fluid dynamics, particularly in relation to control volume calculations and Newton's third law. Participants express confusion about why the reaction force on the nozzle aligns with the flow direction, despite expectations that it should oppose the accelerated mass. Key points include the idea that the resultant force arises from changes in flow direction rather than acceleration, and that pressure differences account for the flow rate without necessitating a resultant force due to acceleration. The conversation also touches on practical implications, such as the need for firefighters to brace hoses against these forces. Ultimately, the dialogue highlights the complexities of interpreting fluid dynamics principles in real-world applications.
  • #51
See this literature article which backs up everything I was saying in this thread: https://link.springer.com/article/10.1007/s10694-018-0719-x Particularly, that there is no reaction force at the nozzle exit and that a straight hose is under tension. Also, it is the curvature of the hose in actual practice that gives rise to what firefighters falsely perceive as a reaction force at the nozzle.
 
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  • #52
I totally agree. In videos, firefighters keep an S shape in the hose, to have the nozzle at waist height. Friction of the length of hose on the ground is probably what provides support against the reaction force you describe.
When a hose is let go it snakes with the motion dictated by an ‘obvious’ force on the curve that forms some distance behind the nozzle.
You have to be fair to firefighters when they accept the story about the nozzle. Newton’s Laws are not uppermost in their minds when wrestling with a live snake.
 
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