Work Energy And Power Question 40
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 [EAMCET 1987; AFMC 2004]
Options:
A) 5
B) 10
C) 11
D) 20
Show Answer
Answer:
Correct Answer: C
Solution:
Let the thickness of one plank is s if bullet enters with velocity u then it leaves with velocity $ v=( u-\frac{u}{20} )=\frac{19}{20}u $ from $ v^{2}=u^{2}-2as $
therefore $ {{( \frac{19}{20}u )}^{2}}=u^{2}-2as $
therefore $ \frac{400}{39}=\frac{u^{2}}{2as} $
Now if the n planks are arranged just to stop the bullet then again from $ v^{2}=u^{2}-2as $
$ 0=u^{2}-2ans $
therefore $ n=\frac{u^{2}}{2as}=\frac{400}{39} $
therefore $ n=10.25 $ As the planks are more than 10 so we can consider n = 11
