Engineering/Algebra
posted by George .
A balsa wood glider wing has the following properties during flight:
Thickness= 0.3175 cm
I= 0.0733 cm^4
Moment= 4.2 Ncm
Find the bending stress in the wing. Express your answer in N/m^2.
Is the answer either 9.096 N/m^2 or 18.192 N/m^2

sigma = M y / Ix
max sigma is at outer fiber top and bottom y = .3175/2 = .159 cm = .00159 m
M = 4.2 N cm * 1 m/100 cm = .042 Nm
I = .0733 cm^4 *^10^2 m/cm)^4 = .0733*10^8 m^4
so
sigma = .042 Nm (.00159m)/.0733*10^8 m^4
= 9.11*10^4 N/m^2 
I see what happened, you used the whole depth of the wing, but the stress due to bending is proportional to the distance from the neutral axis, we assume the middle.
Also you dropped a couple of cm to meter type thingees I think