Q$.4.$ A $650$ N force is applied at the end C of the lever ABC shown in Fig. $2.$ Part AB is of diameter $30$ mm $.$ Determine $($a$)$ the normal and shear stresses on a stress element at point $D,$ whose sides are parallel to the x and y axes, $($b$)$ the principal planes and the principal stresses at D $.$ Ignore the shear stress at D due to shear force of $650$ N $.$

Fig. $2$