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~christina~

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## Homework Statement

An airplane has a mass of 1.60x10^4 kg and each wing has an area of 40.0m^2. During level flight the pressure on the lower wing surface is 7.00x10^4 Pa. Determine the pressure on the upper wing surface.

http://img168.imageshack.us/img168/617/41426717vo3.th.jpg

## Homework Equations

[tex]\sum F= F_L -F_u - F_{mg} = 0 [/tex]

## The Attempt at a Solution

My basic problem is that I keep getting a negative number for the pressure which is odd

[tex]\sum F= F_L -F_u - F_{mg} = 0 [/tex]

[tex] F_u = F_{mg}- F_L [/tex]

P= F/A

[tex] P_u= Mg - P_L A [/tex]

[tex]((9.8m/s^2)*(1.60x10^4kg))-((40.0m^2)(7.00x10^4Pa))= P_u[/tex]

[tex]-2,643,200 = P_u A [/tex]

[tex] P_u= -66,080 Pa [/tex] =>

my problem is why is it negative?? is it because it's force is downward??

Plus..the answer that was given (only the answer) was

**6.80x10^4 Pa**so I must have done something wrong. However I ccalculated it 3 times and I still get the same answer.

Thank you very much =D

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