Net Force on a window

xdarkelf714x

1. Homework Statement
An airplane is flying at an elevation of 12500 m. The window on an airplane is a square of length 0.115 m on each side. What is the net force on the window?

2. Homework Equations
P = Po + pgh
p = 1.26 kg/m^3
Pair = 101.3 kPa

3. The Attempt at a Solution
I thought about using the elevation in the first equation, but I remembered that it applied to the displaced air so I used the height of the window. I had the force from the air pressure on the top of the window and the force from the air pressure on the bottom of the window.

P = 101300 Pa + (1.26 kg/m^3)(9.8 m/s^2)(-0.115 m)
F = P * A
F = 101299 Pa * (0.115 m)^2
F = 1340 N downwards

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PhanthomJay

Homework Helper
Gold Member
You seem to have missing information. You need to know the pressure of the outside air at 12500m, as well as the pressure of the air inside the plane (which is pressurized to a certain level for the comfort and survival of the passengers). If the pressure inside and outside the cabin is the same, there is no net force on the window. it's the difference in pressure that causes the net force. Also, you don't have to compute the small difference in pressure betwen the top and bottom of the window. Also, check your direction of the net force.

xdarkelf714x

Unfortunately, that was the only information I got for the quiz.
You seem to have missing information. You need to know the pressure of the outside air at 12500m, as well as the pressure of the air inside the plane (which is pressurized to a certain level for the comfort and survival of the passengers). If the pressure inside and outside the cabin is the same, there is no net force on the window. it's the difference in pressure that causes the net force. Also, you don't have to compute the small difference in pressure betwen the top and bottom of the window. Also, check your direction of the net force.

PhanthomJay

Homework Helper
Gold Member
Well then i don't know what you are supposed to assume, perhaps the inside of the plane at standard sea level atmospheric pressure? I'm not much into the metric system, but in USA units, the outside air pressure at 12500m (40000 feet or so) might be about only 2psi outside the plane, but maintained at 10 psi inside the plane, resulting in a net outward pressure of 8psi, and a net outward force (directed perpendicular to the window pointed away from the plane) of 8psi times the area of the window.
Or maybe you're supposed to assume the aircraft is unpressurized, in which case I already hinted at the answer.

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