Question

A bullet moving with speed 200 m/s is just able to pierce a wooden block of 6 cm thickness. What velocity is required to just pierce a block of 8 cm?

• A. 400 $\sqrt{3}$ m/s
• B. 400 m/s
• C. 400/ $\sqrt{3}$ m/s
• D. 400 $\sqrt{2}$ m/s

Answer 1

Answer: (A)

400 $\sqrt{3}$ m/s

Answer 2

KE lost by bullet = work done againes plank resistance
$\frac{1}{2}m{{v}^{2}}=Fx$
Here $x$ is the distance pierced
$\frac{v_{2}^{2}}{v_{1}^{2}}=\frac{{{x}_{2}}}{{{x}_{1}}}$
$\frac{v_{2}^{2}}{40000}=\frac{8}{6}$
${{v}_{2}}=\frac{400}{\sqrt{3}}m/s$