Question

A bullet moving with a speed of $100m{s}^{-1}$ can just penetrate into two planks of equal thickness. Then the number of such planks, if speed is double will be:

• A. $6$
• B. $10$
• C. $4$
• D. $8$

Answer 1

The correct option is D $8$

Given that,

Speed of bullet $u=100m/s$

Suppose that, the thickness of one plate is s

Now, from the equation of motion

$v^2-u^2=-2as$

$0-u^2=-2as\left(s\right)$

$u^2\propto s$

Now,

$\frac{v_2^2}{v_1^2}=\frac{s_2}{s_1}$

$\frac{\left({(2\times 100)}^2\right)}{{\left(100\right)}^2}=\frac{s_2}{s_1}$

$s_2=4s_1$

$s_2=4\times 2s$

$s_2=8s$

Hence, the number of such planks is $8$