# Fluid Pressure problem.

• Dec 6th 2009, 05:03 PM
matt1989
Fluid Pressure problem.
Hey all,

I have this problem and I cannot for the life of me figure out what I'm doing wrong that the homework isn't accepting my answer. Can someone take a look at what I'm doing and maybe tell me whats wrong? Problem is below:

A rectangular tank is http://www.ilrn.com/ilrn/formulaImag...+m+%5C%3A&ns=0 wide and http://www.ilrn.com/ilrn/formulaImag...+m+%5C%3A&ns=0 deep. If the tank is only half full with water, find the force against one end of the tank.
(Recall that http://www.ilrn.com/ilrn/formulaImag...%7D%5C%2C&ns=0 = http://www.ilrn.com/ilrn/formulaImag...%7D%5C%2C&ns=0.)

F = ______wN

Pressure on Strip: (6-y)w
Area of Strip: 13dy
Force against strip: (6-y)*13dy = 13w(6-y)dy

F = Integral from 0-3 {13w(6-y)dy}

13w(6y-y^2/2)dy

13w(18-9/2)

13(27/2) = 351/2

I've tried entering my answer in as a decimal and as the exact answer and for some reason its telling me it is incorrect. Any help would be great.
• Dec 6th 2009, 07:56 PM
HallsofIvy
Quote:

Originally Posted by matt1989
Hey all,

I have this problem and I cannot for the life of me figure out what I'm doing wrong that the homework isn't accepting my answer. Can someone take a look at what I'm doing and maybe tell me whats wrong? Problem is below:

A rectangular tank is http://www.ilrn.com/ilrn/formulaImag...+m+%5C%3A&ns=0 wide and http://www.ilrn.com/ilrn/formulaImag...+m+%5C%3A&ns=0 deep. If the tank is only half full with water, find the force against one end of the tank.
(Recall that http://www.ilrn.com/ilrn/formulaImag...%7D%5C%2C&ns=0 = http://www.ilrn.com/ilrn/formulaImag...%7D%5C%2C&ns=0.)

F = ______wN

Pressure on Strip: (6-y)w
Area of Strip: 13dy
Force against strip: (6-y)*13dy = 13w(6-y)dy

F = Integral from 0-3 {13w(6-y)dy}

13w(6y-y^2/2)dy

13w(18-9/2)

13(27/2) = 351/2

I've tried entering my answer in as a decimal and as the exact answer and for some reason its telling me it is incorrect. Any help would be great.

Your method is excellent but w= 10000 seems to have disappeared from you answer! Try [b](351/2)(10000)= 1755000. Gosh, you weren't even close were you?(Giggle)
• Dec 6th 2009, 08:06 PM
matt1989
well the reason why I didn't multiply the 351/2 by 10,000 is because they way you enter the answer you don't have to because they just have the w as part of the answer. Its just a little fill in the blank question and they don't make you multiply your answer by the density of water (10,000). Rather they just make you enter it as 351/2 wN.

But I tried entering in 1755000 too and it still didn't work.

That's why I was wondering if I had done something else wrong.