The block has a mass of $0\mathrm{.}8kg$ and moves within the smooth vertical slot. If it starts from rest when the attached spring is in the unstretched position at $A,$ determine the constant vertical force $F$ which must be applied to the cord so that the block attains a speed $v_B=2\mathrm{.}5\frac{m}{s}$ when it reaches $B$;$s_B=0\mathrm{.}15m.$ Neglect the size and mass of the pulley.