$14\mathrm{.}1-14\mathrm{.}4$

$5.$ Crates $\backslash ($ A $\backslash )$ and $\backslash ($ B $\backslash )$ of mass $50$ kg and $75$ kg $,$ respectively, are released from rest. The linear elastic spring has stiffness $\backslash ($ k$=500\backslash$mathrm$\{$~N$\}/\backslash$mathrm$\{$m$\}\backslash ).$ Neglect the mass of the pulleys and cables and neglect friction in the pulley bearings. If $\backslash (\backslash$mu$\_\{\backslash$mathrm$\{$k$\}\}=0\mathrm{.}25\backslash )$ and the spring is initially unstretched, determine the speed of $\backslash ($ B $\backslash )$ after $\backslash ($ A $\backslash )$ slides $4$ m $.$