A body of mass 50 kg is projected vertically upwards with velocity of 100 m/sec. 5 seconds after this body breaks into 20 kg and 30 kg.  If 20 kg piece travels upwards with 150 m/sec, then the velocity of other block will be                                     

Velocity of 50 kg. mass after 5 sec of projection $\mathrm{v}=\mathrm{u}-\mathrm{gt}=100-9.8\times 5=51\mathrm{m}/\mathrm{s}$

At this instant momentum of body is in upward direction 

${\mathrm{P}}_{\text{intial }}=50\times 51=2550\mathrm{kg}-\mathrm{mis}$

After breaking 20 kg piece travels upwards with 150 m/s let the speed of 30 kg mass is V

 ${\mathrm{P}}_{\text{firal }}=20\times 150+30\times \mathrm{V}$

By the law of conservation of momentum

${\mathrm{P}}_{\text{initial }}={\mathrm{P}}_{\text{fimal }}$

$\Rightarrow 2550=20\times 150+30\times \mathrm{V}\Rightarrow \mathrm{V}=-15\mathrm{m}/\mathrm{s}$

i.e. it moves in downward direction.