What Is the Pressure Inside a Water Cannon Shooting at 25m/s?

[ChickysPusss]

Homework Statement

Water is coming out of a water cannon at 25m/s, what is the pressure inside the cannon?

Homework Equations

Bernoulli's (modifiedd): v = √(2gh) -> h = v^2/2g
Prssure(liquid) : P = ρgh

The Attempt at a Solution

Well, I wasn't too sure how a water cannon works, so I "made one" that is just a giant vat with a lot of water, and a hole at the bottom.

I figured that for the water to come out at 25m/s, the height would have to be ~31m.
h = (25^2)/2g

So then I said, Pressure must be equal to ρgh, which is 1000 * 9.8 * 31m = 303,800Pa

But the answer the book gives says its "4.125 x 10^5 Pa absolute."

Any idea why my solution is incorrect?

 

[mishek]

Absolute pressure? Did You take into consideration atmospheric pressure?

 

[ChickysPusss]

Oh jeeze, I guess that would give me the ~100000 Pascals I need for the answer, thanks!