how to get the formula of shear stress (tau) ? i got (-dp/ds -ydz/ds )(r/2) instead of -d(p+yz)/ ds (r/2) as in the book , if it's d(p+yz)/ ds (r/2) m , i would get (-dp/ds -ydz/ds-z(dy/ds) )(r/2) ,, right ?

how to get the formula of shear stress (tau) ? i got (-dp/ds -ydz/ds )(r/2) instead of -d(p+yz)/ ds (r/2) as in the book , if it's d(p+yz)/ ds (r/2) m , i would get (-dp/ds -ydz/ds-z(dy/ds) )(r/2) ,, right ?

You used the symbol 'y' in your formula, but it's actually gamma. I haven't looked at this very closely, but I assume that gamma is a constant, so your version and the book's versions are identical.

You used the symbol 'y' in your formula, but it's actually gamma. I haven't looked at this very closely, but I assume that gamma is a constant, so your version and the book's versions are identical.