Q: A rifle of 20 kg mass can fire 4 bullets/s. The mass of each bullet is 35 × 10^{-3} kg and its final velocity is 400 m/s. Then, what force must be applied on the rifle so that it does not move backwards while firing the bullets ?

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Sol: Law of conservation of momentum $\large \vec{M V} + \vec{4m v} = 0 $

$\large \vec{V} = -\frac{\vec{4m v}}{M} $

$\large = -\frac{4 \times 35 \times 10^{-3}\times 400}{20} $

= -2.8 m/s

Force applied on the rifle

$\large F = \frac{MV}{t} = -\frac{20 \times 2.8}{1} $

= -56 N