Question

The recoil velocity of a $4.0\, kg$ rifle that shoots a $0.050\, kg$ bullet at a speed of $280\text{ }m{{s}^{-1}}$ is:

• A. $+3.5\text{ }m{{s}^{-1}}$
• B. $-3.5\text{ }m{{s}^{-1}}$
• C. $-\sqrt{(3.5)}\text{ }m{{s}^{-1}}$
• D. $+\sqrt{(3.5)}\text{ }m{{s}^{-1}}$

Answer 1

Answer: (B)

$-3.5\text{ }m{{s}^{-1}}$

Answer 2

${{m}_{1}}{{u}_{1}}+{{m}_{2}}{{u}_{2}}=0$ $\Rightarrow$ ${{u}_{1}}=-\frac{{{m}_{2}}{{u}_{2}}}{{{m}_{1}}}$ $=\frac{0.050\times 280}{4}=-3.5\,m{{s}^{-1}}$