Question

a bullet having a mass 10 g and moving with speed of 1.5m/s penetrate a thik wooden plank of mass 90g.the plank was rest initially.the bullet gets enhanced in the plank and both move together determine their velocity

Answer 1

Your ans is 0.15m/s ...
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Answer 2

Given : 
mass of bullet= 10g= 0.01kg
velocity of bullet= 1.5m/s
mass of plank= 90g=0.09kg
Total momentum before collision=mass of plank x velocity
=0.01x1.5+0.09x0=0.015 kgm/s
Total momentum after collision=(mass of bullet +mass of plank )x velocity
=0.1 kg x V=0.1 V kgm/s
According to law of conservation of momentum,
Total momentum before collision=Total momentum after collision
0.015kg m/s =0.1 v kg m/s
Thus,velocity acquired by the plank and the bullet = 0.15m/s