Physics Fluid

[futureNeuro3]

Why is the answer A? Can someone explain?

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[TheLongGame]

Since the system at point C is exposed to the atmosphere and not bound by the container walls, the pressure of the fluid at that point is equal to atmospheric.

Pabs = atmosphere
Pgauge = 0

You may be wondering "if the pressure is atmospheric, then what is the force driving the water out of the container?" There is most certainly a Pgauge > 0 at all points medial to point C, but once at point C that pressure instantly becomes zero. The fluid continues in its original direction due to momentum, not because the hydrostatic pressure continues to push the fluid.


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[TheLongGame]

Remember that pressure is force/area. So when there are no walls (area) for the fluid to press against (force), there can be no pressure other than that imposed by the atmosphere on the fluid.


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[wizzed101]

are you trying to sabotage people?...

At c the fluid is flowing, when you see this and "ideal" you think Bernoulli's. At b the pressure will be 3 atm (1 atm+ pgh) because the fluid is relatively static. At c however, the pressure will be very close to the point just to the right, which is 1 atm.

 

[TheLongGame]

are you trying to sabotage people?...

At c the fluid is flowing, when you see this and "ideal" you think Bernoulli's. At b the pressure will be 3 atm (1 atm+ pgh) because the fluid is relatively static. At c however, the pressure will be very close to the point just to the right, which is 1 atm.

Not sure what you are upset about. Doing a little math would allow you see that we said the exact same thing.

Hydrostatic pressure

Pabs = Patm + (rho)g(delta Z)

Guess what the gauge pressure is at is at "c"...ZERO! So yes, 1 atm = 1 atm.

However, if you are attempting to counter the fact that a fluid bound by no walls, or any fluid whose surface is exposed to the atmosphere at the point of measurement, has a pressure other than that of the environment, you are incorrect.
Any fluid or gas that is freely exposed to the atmosphere without boundary walls (pipe, piston/cylinder, etc) will instantly assume the pressure of the ambient. The result of seeing water flow out of the hose after it has left the end of the nozzle is due to its inertia and momentum, not because it retains the pressure it had within the hose itself.


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[wizzed101]

?What are you talking about mate?
The reason the OP came up with 3atm in the first place had nothing to do with Pabs or Pgauge, because it is always the absolute pressure that is asked. The only difference between b and c is that the fluid at c is moving while at b it is not.

 

[TheLongGame]

It's simple fluid dynamics!!! In order for a fluid, any fluid, to exert a pressure must have something to push against. No container = no pressure due to the fluid, only the environment which is pushing on the fluid itself.

Now in the drawing we can argue about whether point C is inside the container or outside. My argument is based on point C being at the air/water boundary the moment the plug is removed and the water EXITS the container. Implying that C is external to the container and hence, no container walls means no gauge pressure to add in to the total pressure.

If your point of reference is that C is inside the container, then I agree, there is a hydrostatic pressure that must be added, but it won't be the same as point B. You have to account for the kinetic energy of the now moving fluid. But given the scope and depth of complexity of the MCAT, I would say on the test that the pressure at B = C in this particular case, given a static fluid. Not enough information is given to calculate the exact pressure at point C if it's moving.

Your statement that fluid at point B is static is incorrect. The fluid at point B is moving, just as the fluid level at point A is getting lower and lower as time goes on. No new water is being added to the system, and even if it were, the fluid would still be moving at all identified points.

On the most basic level, we are on the same page. We both understand that the fluid outside the container is at 1 atm. My posts explain why that is the case. Your posts assume that to be already known by the originator, which it obviously was not.

I think this clears it up. Let me know if you're still confused.

 

[wizzed101]

What? Pressure at B=C?
Now you are trolling. In the slim chance you are not, the v in Bernouilli's refers to flow velocity in m/s not volumetric flow rate (m^3/s). At c the velocity is much higher than at A and B.

The stuffs about pressure are so wrong at so many level I don't even bother.

I remember seeing your post the first time on another topic and having a funny feeling that you were trying to sabotage people. Now I am 97% sure that you are a troll.

Shoo!!

 

[TheLongGame]

It's true that Bernoulli's "v" is fluid velocity. How do you think you calculate fluid velocity? Hmmm...perhaps I can derive it from volumetric flow....

Aside from the discussion we are having, let's remember that the OP wants to know why the pressure of the fluid outside the container is at 1 atm. Your posts do nothing to answer their question, nor does your apparent understanding of fluids suggest that you know yourself. The pressure just inside the boundary of the container at the nozzle could be a billion atm, as soon as the fluid crosses the boundary into the environment, the pressure drops to that of the environment.









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[TheLongGame]

You know what? Never mind. Good luck in your future pursuits.


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