How do the magnitude and direction of the principal stress change from point to point within a beam?

When does the plane stress occur?

Why should the moment at the section be equivalent to the moment caused by the stress distribution?

In (Figure 1), w = 3.8 kip/ft and F=20 lb. Follow the sign convention. Part A Determine the shear force acting at point D of the beam. Express your answer in kilopounds to three significant figures. vec VD = kip Figure 1 of 1 Submit Request Answer Part B Determine the moment acting at point D of the beam. Express your answer in kilopound-feet to three significant figures. A, 6 ft 9 ft ΑΣφ vec MD = kip ft %3D