When point B in the above beam sinks by $2\mathrm{.}0$ cm $($but there is no load$),$ what is the magnitude of the force moment $($M$)$ generated at point B$?$ E $=2\mathrm{.}0\backslash$times $10$ kN$/$m$^2,1=0\mathrm{.}05$ m$^2.$