# Doubt from fluids

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1. Jan 24, 2016

### vijayramakrishnan

in fluid statics we have learnt that p =patm+density*g*h.
is the same equation valid in fluid dynamics?

2. Jan 24, 2016

### Dr. Courtney

Look up the Bernoulli equation.

3. Jan 24, 2016

### vijayramakrishnan

thank you for replying sir,but it doesn't provide any direction relation between height and pressure?

4. Jan 24, 2016

### Staff: Mentor

No. The same equation is not generally valid in fluid dynamics. The fluid dynamics of viscous fluids is described by the much more complicated Navier Stokes equations.

5. Jan 24, 2016

If you take a look at it, it's very similar to the equation you provided but also has a velocity term, although it is only valid in a certain subset of fluids problems.

6. Jan 25, 2016

### vijayramakrishnan

thank you for replying sir,but we can assume the fluid to be non viscous and flow to be streamline

7. Jan 25, 2016

### vijayramakrishnan

and also incompressible

8. Jan 25, 2016

### mfig

As generally in physics, dynamic problems and static problems are treated differently. You want to look at this link. Study the three terms of the equation at the top of the page. Read the material there and you will have the answer to your question.

9. Jan 25, 2016

Given your criteria you've now laid out, what was wrong with the Bernoulli answer? Take a look at the link provided by @spamanon.

10. Jan 27, 2016

### vijayramakrishnan

thank you for replying sir,i read that article and it was indeed helpful,and i know what is bernoulli's equation, but all i want to know is can we apply the theorem that pressure = density*g*h in fluid dynamics?for example we have used it in rotating fluids to determine the shape of free surface of liquid but can we do the same if the liquid is moving?or is that equation necessarily not valid in fluid dyanmics?if yes then why so? we use newton's laws to derive that but if the fluid is moving with uniform velocity it has no acceleration so why can't the same law be applied here?

11. Jan 27, 2016

### Staff: Mentor

I'm not sure how many ways people can say "yes" before you will take it, but yes, given all of those constraints and the constant velocity constraint, yes, Bernoulli's equation basically reduces to the hydrostatic pressure equation.

12. Jan 27, 2016

### vijayramakrishnan

13. Jan 27, 2016

You do see that there is a $\rho g z$ term in Bernoulli's equation, right? Bernoulli's equation is essentially an energy balance cast in terms of pressure, where each side of the equation (i.e. each $p + \frac{1}{2}\rho v^2 + \rho g z$ term) constitutes what is commonly called total pressure. One element of that is the hydrostatic pressure.

14. Jan 27, 2016

### vijayramakrishnan

sir,so does the third term represent the hydrostatic pressure?

15. Jan 27, 2016

What do you think and why?

16. Jan 27, 2016

### Staff: Mentor

I don't understand. You were instructed to look up Bernoulli's equation. Did you? Every source describing it should list the meaning of each term. What did your research say?

17. Jan 27, 2016

### Staff: Mentor

Show us what you tried and someone can probably point out where you went wrong,