Answer
Since the diameter is twice the radius, the diameter of the hair is $8.7\times 10^{-5}~m$
Work Step by Step
We can find the radius:
$\frac{F}{A} = 2.0\times 10^8~N/m^2$
$A = \frac{F}{2.0\times 10^8~N/m^2}$
$\pi~r^2 = \frac{F}{2.0\times 10^8~N/m^2}$
$r = \sqrt{\frac{F}{(\pi)(2.0\times 10^8~N/m^2)}}$
$r = \sqrt{\frac{1.2~N}{(\pi)(2.0\times 10^8~N/m^2)}}$
$r = 4.37\times 10^{-5}~m$
Since the diameter is twice the radius, the diameter of the hair is $8.7\times 10^{-5}~m$
