# Homework Help: Pressure at points

1. Feb 26, 2016

### werson tan

1. The problem statement, all variables and given/known data
isn't that p1 and [2 at the same point ? why shouldnt the P1 = P2 , which is
ρg(h1 +h2 +h3) ?
2. Relevant equations

3. The attempt at a solution

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2. Feb 26, 2016

### BvU

The velocity of the liquid makes a difference! Bernoulli.

3. Feb 26, 2016

### werson tan

can you explain further?

4. Feb 26, 2016

### SteamKing

Staff Emeritus
BvU means you must apply the Bernoulli equation to the two points to find out what the pressures are at those locations.

When the fluid is in motion, the pressure at any point is no longer equal to just the static pressure.

5. Feb 26, 2016

### werson tan

can you explain how to apply the Beroulli's pronciple so that the P1 = pg(h1 +h2) ? but not P1 = pg(h1 +h2+h3) ???

6. Feb 26, 2016

### BvU

Have you learned about Bernoulli already ? This is a very straightforward application!

7. Feb 26, 2016

### werson tan

maybe there are some points that i have left out , can you point out please?

8. Feb 26, 2016

### Staff: Mentor

Please write out the Bernoulli equation for us (if you know it)?

9. Feb 26, 2016

### werson tan

P +z + (v^2) /.2g , wher v = velocity

10. Feb 26, 2016

### Staff: Mentor

Please try again. This is not an equation. I know this, because I don't see an equal sign. I also don't see any density in the equation. None of the terms in the equation are dimensionally consistent with one another. Please look it up and get it right this time. Otherwise, you won't be able to solve your problem.

11. Feb 26, 2016

### werson tan

it should be P/ y + z + (v^2) /.2g , where v = velocity , y = ρg

12. Feb 26, 2016

### BvU

This is almost comical: 0084.png in post #1 says it all but apparently isn't acceptable for Werson. Do you find it difficult to understand the term with $v^2$ in the Bernoulli equation ?

(What Chet means is that this expression is a constant)

13. Feb 26, 2016

### Staff: Mentor

This would be correct if that expression were set equal to a constant, and if that 0.2 in the denominator were a 2. Do you agree?

14. Feb 26, 2016

### werson tan

how to relate it to the P1 ? why not P1 = P2 , which is ρg(h1 +h2 +h3) ?

15. Feb 26, 2016

### Staff: Mentor

Let's try to apply the Bernoulli equation to P1 and P2 to see what it tells us about their relationship. But, before doing that, do you understand the Bernoulli equation? Also, precisely what does z represent in the Bernoulli equation?

16. Feb 26, 2016

### werson tan

z represent the difference in height between 2 points

17. Feb 26, 2016

### Staff: Mentor

You should think of z more as the elevation above a specified datum z = 0. In this problem, a logical choice for the datum would be the center of the pipe. Now please articulate what the Bernoulli equation means to you, or what each of the terms in the equation means physically.