A bullet loses ` 1//20` of its velocity in passing through a plank. What is the least number of plankd required to stop the bullet .

Options

A. 5
B. 10
C. 11
D. 20

Answer

C

Explanation

`k_i=1/2 mv^2`
For one plank `rarr K =W`
`W=DeltakimpliesW= underset("Loss inK.E.")(underbrace(1/2mv^2-1/2m((19v)/(20))^2))`
`implies W=1/2 mv^2 [1-((19)/20)^2]`
n = Number of plank
`n=(1/2mv^2)/(W)=1/([1-((19)/20)^(2)])=((20)^2)/((20)^2-(19)^2)=400/(39)`
`n = 10.25 ~~ n = 11`