20.A bullet loses 1/20 of its velocity on passing through a plank. The least number of planks required to stop the bullet is

solution:

velocity of bullet after passing from plank ,its velocity decreases .

initial velocity(u)=u

final velocity(v)=initial velocity-1/20 of initial velocity

v=u-u/20

v=19u/20

we have formula

for one plank,

(v)²-(u)²=2as,

(19u/20)²-(u)²=2as

39u2/400=2as ……….(1)

for n planks:

then width of one plank will be ‘s’

width of n planks will be ns

after passing from n planks ,the velocity will be zero

(0)²-(u)²=2ans,

-(u)²=2ans,

n=-(u)²/2as=-(u)²/39u2/400=11

The number of planks required to stop the bullet is 11