Find Magnitude of F to Cause Net Effect of 3 Forces: Stress in Bolt

[mert]

Homework Statement

(a) By using either the vector algebra or polygon methods for finding a
resultant, determine the magnitude of F that will cause the net effect of
the three forces to act vertically.
(b) For that value of F, determine the stress in the bolt’s 1-cm diameter
straight shank.

Homework Equations

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The Attempt at a Solution

I have found that the sum of the vertical forces are 791.75 N. But I don't know how to calculate the tensile stress in a bolt, is there a special equation to find the area? (sorry I'm new at mechanical engineering :) )
By the way the answers are: (a) 568.46 N, (b) 10.08 MPa

 

[mert]

I found the answer,
The cross sectional area is A= (pi*d^2)/4 = 0.00000785398 m^2
791.75/0.0000785398= 10080876.1927 Pa = 10.08 MPa

But I don't understand why do we divide the area by 4?

 

[CWatters]

Look up the definition of stress, or even just the units...

http://en.wikipedia.org/wiki/Stress_(mechanics )

Units
The dimension of stress is that of pressure, and therefore its coordinates are commonly measured in the same units as pressure: namely, pascals (Pa, that is, Newtons per square metre) in the International System, or pounds per square inch (psi) in the Imperial system.

 

[CWatters]

But I don't understand why do we divide the area by 4?

The area of a circle is Pi*R^2 or Pi*(D/2)^2 = (Pi*D^2)/4

 

[mert]

Oh sorry, i didn't realize that it was the diameter, I tried to solve the problem like it was the radius. My bad :) (Sorry for my poor english) The reason i couldn't solve the problem at the first place is i overlooked the word diameter, thanks