Help with a pipe, water, continuity and Bernoullie

[TFM]

[SOLVED] Help with a pipe, water, continuity and Bernoullie

Homework Statement

At one point in a pipeline the water's speed is: 3.0 m/s,
the gauge pressure is: 5.0*10^4.

Find the gauge pressure at a second point in the line, 11m lower than the first, if the pipe diameter at the second point is twice that at the first.

Homework Equations

Bernoullies Equation: P1 + density*g*y1 + 0.5*density*v1^2 = P2 + density*g*y2 + 0.5*density*v2^2

Continuity Equation: A1V1 = A2V2

The Attempt at a Solution

I work out the speed of v2 using continuity, with area-1 being pi*r^2, Area-2 being pi*4r^2,
This makes the speed 3/4 m/s

I themn put all known variables into Bernoullies equation, which gives me a negative pressure! (-53581.25) this cannot be right?

Any Ideas/Suggestions

TFM

 

[Doc Al]

I work out the speed of v2 using continuity, with area-1 being pi*r^2, Area-2 being pi*4r^2,
This makes the speed 3/4 m/s

Looks good.

I themn put all known variables into Bernoullies equation, which gives me a negative pressure! (-53581.25) this cannot be right?

No, it can't. What did you put for y1 & y2?

 

[TFM]

I first put y1 as 0, y2 as 11, then tried y1 as 1, y2 as 12

It should be y2 = -11, shouldn't it? -goves an answer of 162018.75

 

[Doc Al]

It should be y2 = -11, shouldn't it?

Yep.

 

[TFM]

162018 was the right answer

Thanks

TFM