Question

In the bending equation sigma=Mc/I, what is c?  

I understand everything about this problem except for the very last step. What is c and how do I find it?

Answer 1

Consider the cross section of the beam,

Explanation:
Draw the given figure representing the location of neutral axis from the bottom of the beam and also the point c.

Write the location of the neutral axis from the bottom of the beam (given in the question).

Here, the perpendicular distance from the outermost fiber to the neutral axis is c.

Explanation:
After calculating the location of the neutral axis from the bottom of the beam, the value of c can be calculated by subtracting location of the neutral axis from the total height of the beam. Here, the value of c is the maximum distance from the neutral axis to the outermost fibre of the beam. If the location of the neutral axis is calculated from the top of the beam, then it equals to c instead.