Question

A rifle of $20\, kg$ mass can fire $4$ bullets per second. The mass of each bullet is $35 \times 10^{-3}\, kg$ and its final velocity is $400\, ms ^{-1}$. Then what force must be applied on the rifle so that it does not move backwards while firing the bullets?

• A. 80 N
• B. 28 N
• C. -112 N
• D. -56 N

Answer 1

Answer: (D)

From conservation of momentum
$M V+(4 m v)=0$
$\Rightarrow V=-\frac{4 m v}{M}=-\frac{4 \times 35 \times 10^{-3} \times 400}{20}$
$=-2.8\, ms ^{-1}$
Force applied on the rifle,
$F =\frac{M V}{t}=-\frac{20 \times 2.8}{1}$
$=-56\, N$