# Moment of inertia in shear stress formula

1. Oct 28, 2016

### foo9008

1. The problem statement, all variables and given/known data
In the first photo , i was told that the I represent the moment of inertia about the neutral axis ... But , in the 2nd photo , i found that the author calculate the moment of inertia about the x-axis (parallel to neutral axis ) and y -axis ( perpendicular to neutral axis) ...

2. Relevant equations

3. The attempt at a solution
So , is the example in the second photo wrong ?

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2. Oct 28, 2016

### PhanthomJay

Not sure what you mean. The moment of inertia is correctly calculated about the horizontal neutral axis of the beam's cross sectional area. If you call the x axis as the axis along the beam's 8 m length, and the y axis as the vertical axis, then the horizontal neutral axis in question lies along the z axis into the plane of your screen.

3. Oct 28, 2016

### foo9008

sorry , i misunderstood something . Clear now .

For second question , i let the horizontal axis as x-axis ( which is also the neutral axis) ...So , the moment of inertia is about the neutral axis , which is only Ixx only , right ? We dont have to consider Iyy ( moment about vertical axis , right ) ?

4. Oct 28, 2016

### PhanthomJay

That is correct, because the bending moments are about the horizontal neutral axis and shears are along the y axis. If the moments were about the vertical neutral axis (loading into the plane of the screen) then you would have to consider Iyy. This is not the case here.

5. Oct 28, 2016

### foo9008

I defined y axis as the axis that is up , which is not into the book ....

6. Oct 28, 2016

### PhanthomJay

You don't have to worry about the y axis. Not for this problem. If applied loading was into the book, only then would you need to consider weak axis bending about the vertical y axis.