# Fluid Mechanics - Pressure - U-Tube Manometer

1. ### Aerospace

39
A piston having a cross-sectional area of 0.07 m^2 is located in a cylinder containing water as shown in the figure. An open u-tube manometer is connected to the cylinder as shown. For h1=60 mm and h=100 mm, what is the value of the applied force, P, acting on the piston? The weight of the piston is negligible.

I'm confused because there is more than one "u"-shape in the figure, so, it's unclear to me how to proceed.

Help would be appreciated!

legend for figure :
p = piston
first weird looking h = h1
second weird looking h = h
piston is present on top of the water.
Force P downwards on piston.

File size:
5.6 KB
Views:
128
2. ### OlderDan

3,030
The U on the right is the manometer. I've never seen a manometer filled with water on one side, but that's the way your diagram looks. Is there any air between the water and the mercury? It's not clear to me why the 60mm is given, but the height of the water relative to the top of the mercury is important

3. ### Aerospace

39
Here's a scan of the piston I'm talking about.

File size:
24.9 KB
Views:
106

### Staff: Mentor

This fact may help: Since the fluid is connected (and in hydrostatic equilibrium) the pressure at any height within the same fluid must be the same throughout.

Last edited: Jun 19, 2005
5. ### Aerospace

39
Ok, so far this is what I've got. Let me know if I'm on the right track.

If you take the pressure at the piston as P1 then,
P1 + $$\gamma$$H2O*h1 - $$\gamma$$Hg*h = 0

Solve for p1 and then
P(Force)= p1*Area

Last edited: Jun 19, 2005

### Staff: Mentor

Don't forget that the tube is open to the air. (But you are on the right track.)

7. ### Aerospace

39
Hmmm. I did take that into account when I did -$$\gamma$$(Hg)*h

### Staff: Mentor

That's the pressure due to the column of mercury. But what about the air on top of the mercury?

9. ### Aerospace

39
well, isn't the pressure at the open end supposed to be equal to zero?

### Staff: Mentor

Only if it's in a vacuum.

11. ### Aerospace

39
hmmm then what would it be? because my book says "at the open end the pressure is zero."

12. ### OlderDan

3,030
This is a matter of interpretation. If the book says zero, they intend for you to find only the extra force applied to the piston that is added to the force of air pressure already acting. Air pressure is the same on both sides, so it can be neglected if all you need is the extra force.

Don't forget to calculate the force. If all you have done is caclulate the added pressure, you are not finished.

13. ### Aerospace

39
so the force P = p1*A, correct?
I stated that above in my post...is this incomplete?

And in any case, if Pair was not neglected, how would you write it into the equation?

Last edited: Jun 19, 2005

3,030