When the bending moment M$=50$kNm acts on the rootless concrete beam section with a width b$=300$mm and a depth of $600$mm$,1.$ Find the distance from the compression podium to the neutral axis and the distance from the tensile podium to the neutral axis$2.$ Find the second moment of the cross section with respect to the neutral axis$3.$ Find the vertical stress of the compression end and the vertical stress of the tensile end caused by the bending moment$4.$ When the tensile strength of concrete is $2$ Mpa, determine if cracks occur in the beam section