In Fig, shown, both blocks are released room rest. Length of 4 kg block is 2 m and of 1 kg is 4m. Find the time they take to cross each other ? Assume pulley to be light and string to be light and inelastic.  ( $g=10m/s^2$)
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From FBD of blocks A and B solve acceleration of each block   $4g-T=4a$….. (1)
 $T-1g=1\times a\mathrm{.........}\left(2\right)$
After solving  eqns (1) and (2) ,  $a=\frac{3g}{5}$
Acceleration of A W.r.t B  $a_{A/B}=\frac{6g}{5}=12m/s^2$
If A will cross B then distance travelled by A w.r.t B is 6 m
  $6=0+\frac{1}{2}\times 12\times t^2,t=1\sec$