At a point in a rectangular block of elastic material under strain, there is a tensile stress of 50MPa on one plane and a compressive stress of 30 MPa on a plane at right angles. These stresses are accompanied by shearing stress of 30 MPa. The shear stress performs on the plane carrying tensile stress in such a way that it goes to rotate the block anticlockwise. Draw the block with system of stresses and calculate analytically and graphically the following :

(a) the value of principal stresses

(b) the direction of principal planes

(c) the magnitude of greatest shear stress.