# Homework Help: Beam (compression vs tension)

1. Jul 4, 2016

1. The problem statement, all variables and given/known data
I have the bent beam like this . The inner part of beam undergo compression , right , so it shortens. Why the stress is minimum at here ?
the outer part undergo tension , why the normal stress is maximum here ?
This is from my notes
http://imgur.com/a/H8fy4
2. Relevant equations

3. The attempt at a solution
since the outer part elongate , the surface area is max , so it should has min stress , right ? pressure = force / area

2. Jul 4, 2016

### SteamKing

Staff Emeritus
Bending stress is not equal to force / area.

The terms "minimum stress" and "maximum stress" are entirely relative in beam bending.

3. Jul 4, 2016

can you explain why bending stress is min when the beam is compressed?

4. Jul 4, 2016

### Staff: Mentor

Analysis of the kinematics of deformation shows that on the outside of the bend (relative to the neutral axis), the axial strain is positive, and, on the inside of the bend (relative to the neutral axis), the axial strain is negative. At the neutral axis, the axial strain is zero. Along with these axial strains go axial stresses, which are proportional to the axial strains. So the axial stresses are positive on the outside of the bend (tension), and the axial stresses are negative on the inside of the bend (compression).

5. Jul 4, 2016

so compression correspnds to negative(minuimum stress) tension corresponds to positive(maximum stress) ??

6. Jul 4, 2016

Sure.