Question

A bullet travelling horizontally loosess 1/20th of its velocity while piercing a wooden plank. Number of such planks required to stop the bullet is ?

Answer 1

applying v^2=u^2+2ad  ,here d is the width of plank

 v=u/20 after penetrating 0ne plank

  2ad=399u^2/400 .........eq1 

  for n number of planks total distance after which bullet stops is nd

 again applying v^2=u^2+2adn  and putting v=0

  n=u^2/2ad......eq2

 solving 1and 2 

   n=1

Answer 2

velocity looses = 1/20th

1/n=1/20

n=20

no.of planks = n²/2n-1

= (20)²/2(20)-1

=400/40-1

=400/39

=10.25

approximately 11