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Question

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


A

5

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B

10

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C

11

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D

20

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Solution

The correct option is C

11


Let the thickness of one plank is s

v=(uu20)=1920u

from v2=u22as (1920u)2=u22as40039=u22as

Now if the n planks are arranged just to stop the bullet then

again from v2=u22as

0=u22ans

n=u22as=40039

n=10.25

As the planks are more than 10 so we can consider n = 11


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