$AB{A}^{\text{'}}{B}^{\text{'}}$ is a segment of a beam of rectangular cross$-$section, $200mm$ deep and $100mm$ wide. The length of the segment is $500mm.$ There are no external loads on the segment, but there are bending moments at $A{A}^{\text{'}}$ and $B{B}^{\text{'}}$ which produce the linear stress distribution shown in Fig. $3,$ as well as a shear force on AA and BB$\text{'}.$

$($a$)$ Using the basic principles of equilibrium determine the shear forces on the faces $A{A}^{\text{'}}$ and $B^{\text{'}}\text{'}.$

$($b$)$ Take as freebodies those parts of the segment above and below the neutral axis, and determine by statics the horizontal shear force on the horizontal mid$-$section of the beam.

$($c$)$ For both the horizontal and vertical shear forces, calculate the ratio of shear force to the area on which it acts.

$^$ ABA'B' is a segment of a beam of rectangular cross section, $200$mm deep and $100$mm wide the length of segment is $500$mm$.$

Could you explain how for the first part to find shear force u multiply $8*100*100*1/2.$and for force b u multiply $10*100^2*1/2$ How does this formula for shear force occur and why do we multiply by $100$ twice? How is the moment calculated. the answer