Moment of Normal Force:

Now, let us discover the moment of this normal force around the neutral axis. Again from Figure .

Normal force on the elemental area = σ_max  ( y/ y_max) da

                                                         = ( σ_max / y_{max )} y da

Moment of this normal force around the neutral axis =( σ_max / y_{max )}  y²  da

∴ Moment of normal force on the shaded area around the neutral axis

= ∑( σ_max / y_max) y²  da

=  (σ_max / y_max ) ∑ y²  da

But ∑  y ²  da =  Moment of inertia of the shaded area around the neutral axis, Is.

∴ Following is Moment of normal force on a partial beam section around the neutral axis,

                                                  = (σ_max/ y_max )× I _s