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2015-05-01T14:39:43+05:30

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Thickness of plank =  d
initial velocity of bullet = u
final velocity = v = u - u/20 = 19 u / 20

deceleration of the plank = a
      v² - u² = 2 a s
        =>      a = [ (19 u/20)²  -  u² ] / 2d
         =>    a = - 39 u² / (800 d)
 
  Let  v = 0  and  then  the distance to travel inside planks
      s = [ 0² - u² ] / 2 a  = 800 d / 39

   =>  s/d =  800/39 = 20.5

So we need 21 planks to stop the bullet.

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