Q. 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 $400\ {ms}^{- 1}.$ 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
Explanation
(d) From conservation of momentum
$MV + (4mv) = 0$
$\Longrightarrow \ \ \ \ V = - \frac{4mv}{M}$
$= - \frac{4 \times 35 \times 10^{- 3} \times 400}{20}$
$= - 2.8\ {ms}^{- 1}$
Force applied on the rifle,
$F = \frac{MV}{t} = - \frac{20 \times 2.8}{1} = - 56\ N$
Reviewed by Dr. Abubacker
Physics Educator & NEET/JEE Specialist
Ph.D. in Physics with 19+ years of teaching experience.