Question

A bullet loses 1/20 of its velocity on passing through a plank . The least number of planks required to stop the bullet A 10 B. 11. C 12. D 13 Correct answer is With solutions

Answer 1

Hi friend,

# B. 11

Let the thickness of one plank = d
and the acceleration provided by the plank = a

V^2 = vo^2 + 2ad
if n plank are required to stop the bullet , than
0^2 = vo^2 + 2a*nd
2and = -vo^2
n= vo^2 / (-2ad)---------------(1)

V= vo - vo/ 20 = 19 vo/ 20 in passing through one plank.
( 19 vo/20 ) ^2 = vo^2 + 2ad
361 / 400 * vo^2 = vo^2 + 2ad
-2ad = vo^2 ( 1 - 361/400 )
-2ad = vo^2 * 39 / 400

substituting the valu of -2ad into equation ;-

n = vo^2 ( vo^2 * 39/400 ) = 400/39


The minimum no. of planks needed = the smallest Integer greater than 400/39 = 11

Ans :- 11 ( proved )

....i hope it helps you


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