Question

In a ballistics demonstration a police officer fires a bullet of mass $50g$ with speed $200m{s}^{-1}$ on soft plywood of thickness $2cm$. The bullet emerges with only $10$% of its initial kinetic energy. The emergent speed of the bullet is:

• A. $2\sqrt{10}m{s}^{-1}$
• B. $20\sqrt{10}m{s}^{-1}$
• C. $10\sqrt{2}m{s}^{-1}$
• D. $10\sqrt{20}m{s}^{-1}$

Answer 1

Answer: (B)

$20\sqrt{10}m{s}^{-1}$

$\frac{1}{2}m{v}^2=0.1\left(KE\right)$

$=0.1\frac{1}{2}m{u}^2$

$\frac{1}{2}\times 50\times {10}^{-3}{u}^2=0.1\times [\frac{1}{2}\times 50\times {10}^{-3}\times {20}^2]$

$u^2=0.1\times {200}^2$

$u=20\sqrt{10}$