b
સ્થિતિ ઉર્જા માં વ્યય = ગતિઉર્જા માં વધારો 

\(mg\,\,\,\frac{\ell }{3}\,\, + \,\,mg\,\,\left( {\frac{{2\ell }}{3}} \right)\,\, + \,\,mg\ell \,\,\)

\( = \,\frac{1}{2}\,\,\left( {m\,\,{{\left( {\frac{\ell }{3}} \right)}^2}\, + \,\,m\,\,{{\left( {\frac{{2\ell }}{3}} \right)}^2} + \,\,m{\ell ^2}} \right)\,\,{\omega ^2}\)

\( \Rightarrow \,\,\,\omega \,\, = \,\,\sqrt {\frac{{36g}}{{14\ell }}} \,\,\, \Rightarrow \,\,\,{v_B}\, = \,\,\,\omega {\ell _B}\)

\(\, = \,\,\frac{{2\ell }}{3}\,\,\sqrt {\frac{{36g}}{{14\ell }}} \,\,\, = \,\,\sqrt {\frac{{8g\ell }}{7}} \)