The end A of a thin rod AB of mass m and length l is hinged and the end B is attached to X string BC hanging from a ceiling. Just after the string is burnt

Just after the string is burnt, weight mg produces a moment about the hinge and clockwise angular acceleration α=mg l213ml2=3glSo a vertically upward pseudo force m.αl2 acts on the rod as shown in figure. Because of the action of this pseudo force, N becomes less than mg.