A) 5
B) 10
C) 11
D) 20
Correct Answer: C
Solution :
Let the thickness of one plank is s if bullet enters with velocity u then it leaves with velocity \[v=\left( u-\frac{u}{20} \right)=\frac{19}{20}u\] from \[{{v}^{2}}={{u}^{2}}-2as\] Þ \[{{\left( \frac{19}{20}u \right)}^{2}}={{u}^{2}}-2as\]Þ \[\frac{400}{39}=\frac{{{u}^{2}}}{2as}\] Now if the n planks are arranged just to stop the bullet then again from \[{{v}^{2}}={{u}^{2}}-2as\] \[0={{u}^{2}}-2ans\] Þ \[n=\frac{{{u}^{2}}}{2as}=\frac{400}{39}\] Þ \[n=10.25\] As the planks are more than 10 so we can consider n = 11You need to login to perform this action.
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