At a point in an elastic material, there are two mutually perpendicular planes, one of which carries a direct tensile stress of 50N/mm^2 and a shear stress of 40N/mm^2, while the other plane is subjected to a direct compressive stress of 35N/mm^2 and a complementary shear stress of 40N/mm^2. Required:
Determine the principal stresses at the point, the position of the planes on which they act, and the position of the planes on which there is no normal stress.