Force Acting on Area: ucos(theta)dA vs usin(theta)dA

[foo9008]

Homework Statement

dA lies on x and y aixs , right ? the author gave that the volume flow rate thru element A is ucos(theta)dA , why not
usin(theta)dA ? if u ' = ucos(theta) , it will become parallel to dA , right ? since u' will be parallel to x-axis...

Homework Equations

The Attempt at a Solution

 

[BvU]

dA is in the yz plane