JEE Main & Advanced Physics Motion in a Straight Line / सरल रेखा में गति Question Bank Topic Test - One Dimensional Motion

A rifle bullet loses \[\frac{1}{4}\] of its velocity in passing through a wooden plank. The least number of planks required to stop the bullet is (assume uniform retardation in the plank)

A) 4

B) 5

C) 2

D) 3

Correct Answer: D

Solution :

[d] Let s be the thickness of plank and a the retardation. Then
\[r\text{ }\alpha \text{ }{{n}^{2}}\]	....(1)
and \[\upsilon \text{ }\alpha \text{ }\frac{1}{n}\]	....(2)
\[2as=\frac{7}{16}{{V}^{2}}\]
Substituting in Eq. (2)
\[n=\frac{{{V}^{2}}}{2as}=\frac{16}{7}=2.28\]
Thus number of planks required is 3.