Holding nozzle stationary? (applications of COM, bernoulli's,mass flow

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Discussion Overview

The discussion revolves around the feasibility of holding a nozzle stationary under certain conditions, referencing concepts from conservation of momentum, Bernoulli's principle, and mass flow. Participants analyze a specific problem related to the forces involved and the practical implications of holding the nozzle in place.

Discussion Character

  • Debate/contested
  • Technical explanation
  • Conceptual clarification

Main Points Raised

  • One participant questions the ambiguity of the answer stating that a person could hold the nozzle stationary, asking if this depends on their weight.
  • Another participant provides a different force calculation of 8344 N (approximately 1875 lbs), asserting that while a person could not hold it, mechanical means could.
  • A third participant interprets the original force calculation of about 159 lbs, suggesting that if their weight exceeds this, they could hold it stationary, and if less, they could not.
  • A later reply recounts personal experience with a fire hose of similar size and pressure, indicating that while it can be held by a strong person, it requires significant effort and is not sustainable for long durations. They also note the potential for the hose to whip, which could increase the effective force needed to hold it.

Areas of Agreement / Disagreement

Participants express differing views on the feasibility of holding the nozzle stationary, with no consensus reached on the conditions under which it could be done. Some argue it is possible under certain weight conditions, while others contend it is impractical without mechanical assistance.

Contextual Notes

Participants reference specific calculations and personal experiences, but there are unresolved assumptions regarding the conditions of holding the nozzle, such as the effects of friction and the dynamic nature of the forces involved.

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Holding nozzle stationary? (applications of COM, bernoulli's,mass flow"

http://baen.tamu.edu/users/gilley/BAEN%20340%20Fluid%20Mechanics/Handout%20Items/Example%20Momentum%20Problem%20sets.pdf

The problem is shown on page 12 of ^ link. It also shows the solutions. However the question asks could you hold the thing stationary. explain.

The answer is supposed to be yes.

They calculated the force to be about 159lb. This answer "yes" seems ambiguous. Why can the person hold the force stationary?

Does it depend on how much I weigh?
 
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They calculated the force to be 8344 N. That's about 1875 lbs. Obviously a person could not hold it but it certainly could be held by mechanical means.
 


No, the problem on page 12. They calculated it to be about 159lbs. If I weigh >159lbs, I can hold it stationary anything less and I can't. Is that how to interpret it?
 


Sorry, I was looking at the wrong problem. I and another person have held a fire hose of that size and about that pressure. It can be held by one person if he is a strong person but it takes large effort. You would not hold it for a long time so you'd better put the fire out fast. So, in general, my answer would be no.

The hose also has a tendency to whip. Since the load then is a moving one, the force can become greater than 159 pounds at times. When you consider friction between you and the ground, it would take more than one person to hold it.
 
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