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

In summary, the conversation discusses the application of the principles of center of mass, Bernoulli's equation, and mass flow in the context of holding a fire hose stationary. The group agrees that it is possible for a person to hold the hose stationary, but it would require a large amount of effort and is not a sustainable solution. They also mention the potential for the hose to whip and the impact of friction on the force needed to hold it.
  • #1
pyroknife
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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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  • #2


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.
 
  • #3


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?
 
  • #4


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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  • #5


I would say that yes, the person's ability to hold the force stationary would depend on their weight and strength. In this problem, the force being exerted by the water through the nozzle is balanced by the force exerted by the person holding the nozzle. This is an example of Newton's Third Law of Motion, which states that for every action, there is an equal and opposite reaction.

In order for the person to hold the nozzle stationary, they would need to exert a force equal to or greater than the force being exerted by the water. This force would need to be applied in the opposite direction, in order to balance out the forces and keep the nozzle from moving.

This situation also involves the principles of center of mass (COM), Bernoulli's equation, and mass flow. The center of mass of the system (person + nozzle) must remain in a stable position in order for the person to hold the nozzle stationary. Bernoulli's equation relates the pressure, velocity, and height of the water as it flows through the nozzle, and helps to explain the force being exerted by the water. Mass flow is also important, as it determines the amount of water flowing through the nozzle and the force it exerts.

In conclusion, the ability to hold the nozzle stationary depends on the person's weight and strength, as well as the principles of COM, Bernoulli's equation, and mass flow.
 

1. How does holding the nozzle stationary affect the center of mass (COM)?

When the nozzle is held stationary, the center of mass remains in the same position as it would if the nozzle was in motion. This is because the motion of the nozzle does not affect the distribution of mass and the location of the center of mass.

2. What is the role of Bernoulli's principle in holding the nozzle stationary?

Bernoulli's principle states that as the velocity of a fluid increases, the pressure decreases. In the case of holding a nozzle stationary, the velocity of the fluid exiting the nozzle is zero, resulting in a higher pressure at the nozzle exit. This principle is important in applications such as airfoil design and fluid dynamics.

3. How does holding the nozzle stationary affect mass flow?

When the nozzle is held stationary, the mass flow remains constant. This is because the mass flow is determined by the cross-sectional area of the nozzle and the velocity of the fluid, both of which remain unchanged when the nozzle is stationary.

4. What are some practical applications of holding the nozzle stationary?

Holding the nozzle stationary is commonly used in industries such as aerospace, automotive, and chemical engineering. It is used in applications such as rocket propulsion, fuel injection, and spray nozzles.

5. Are there any disadvantages to holding the nozzle stationary?

One disadvantage of holding the nozzle stationary is that it can lead to pressure build-up and potential clogging in the nozzle. This can be mitigated by periodically releasing the nozzle or using alternative designs such as rotating nozzles.

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