# Homework Help: Pressure difference

1. Mar 5, 2016

### foo9008

1. The problem statement, all variables and given/known data
why the author ignore the pressure difference at point 1 and point 2 in the energy equation ?pressure at pont 1 must be higher than pressure 2 , right ? as the height of P1 is higher

2. Relevant equations

3. The attempt at a solution

2. Mar 5, 2016

### foo9008

notes attached here

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3. Mar 5, 2016

### haruspex

Pressure and energy are dimensionally different. How would you propose to take pressure into account for an energy equation involving incompressible fluid?

4. Mar 5, 2016

### foo9008

sorry , why the author did not take the P/ y into the calculation ? where y = ρg

5. Mar 5, 2016

### haruspex

The 2+ and 1+ are the potential energy terms. It is a bit confusing because the author has divided out ρg everywhere.

6. Mar 5, 2016

### foo9008

after dividing the ρg why there is no P to be taken into the calculation ? the author assume P1 and P2 as same pressure? why ?

7. Mar 5, 2016

### haruspex

Bernoulli's equation refers to identified points in the streamline. The author's choice of height values (1, 2) indicates the points being chosen are on the surface, so the pressure is the same. If instead you pick points at the bottom of the stream then the heights are the same and the pressures are different, but the equation turns out the same.

8. Mar 5, 2016

### foo9008

so do you mean the author choose point 1 and2 on the surface??

9. Mar 5, 2016

### haruspex

It seems that way.