# Homework Help: Hydrodynamics: Pressure of water coming out of a glass

1. Dec 6, 2009

### qnney

"What pressure do you need to get water to flow at 2 m/sec coming out of a hole?"

Here is the visual of a container sitting on top of a glass of water with a hole poked through the bottom: http://i96.photobucket.com/albums/l168/synovial/fluids.jpg

Given: A1, A2, V2, (y1-y2)

Find: patm

Useful equations:
Bernoulli's Equation:
p1 +$$\rho$$ $$\cdot$$ g $$\cdot$$ y1 + 0.5 $$\rho$$$$\cdot$$ V12 = p1 +$$\rho$$ $$\cdot$$ g $$\cdot$$ y2 + 0.5 $$\rho$$$$\cdot$$ V22

A1V1 = A2V2

I'm not sure how to modify the Bernoulli's equation to combine the y1 and y2 into (y1 - y2) and to also combine the pressures into patm. Can someone please help me?

Last edited: Dec 6, 2009
2. Dec 6, 2009

### denverdoc

Bernoullis is just a statement of conservation of energy.

The potential energy is the static pressure = rho*g*h where rho is density and the
kinetic energy =1/2pho*v^2.

Look familiar? So we convert from one form to the other.

Almost like a kinematics problem.

3. Dec 6, 2009

### qnney

I'm not sure I understand. Is the pho supposed to be rho or p*rho?

4. Dec 6, 2009

### denverdoc

rho throughout. But as it turns out it is a common factor leaving 2gh=v^2. As I said, does this look familiar?

5. Dec 6, 2009

### qnney

Yes it does... especially the 2gh=v^2.

One last question: do I need the v^1, or can I find it from what is given?

6. Dec 6, 2009

### denverdoc

Well if you mean v^1 is velocity, yes that is what we are trying to solve for as in equal to 2m/second. So yes you need to take the square root if that is what you are asking.

7. Dec 6, 2009

### qnney

Yes... thank you!