A rifle bullet loses 1/20th of its velocity in passing through a plank. What will be the least number of such planks required to just stop the bullet ?

Explanation

For one plank
u=u, `v=u-u/20=19/20u`
`((19v)/20)^2-v^2=2ax`…(1)
For n planks
u=u, v=0
`0^2 -v^2=2anx`…(2)
Dividing (2) , (1) `n=(-v^2)/((19/20v)^2-v^2)=1/(1-(19/20)^2)`
`=(20xx20)/((20+19)(20-19))=400/39` =10.3
`rArr` so, 11 planks are required. The bullet shall stop in 11th plank