From a rifle of mass 4 kg,

Solution:

Mass of rifle $=4 \mathrm{~kg}$

Let velocity of recoil of rifle $=\mathrm{v}$

Momentum of the rifle $=4 \times \mathrm{v}$

Mass of bullet $=50 \mathrm{~g}=\frac{50}{1000} \mathrm{~kg}=0.05 \mathrm{~kg}$

Velocity of bullet $=35 \mathrm{~ms}^{-1}$

Momentum of bullet $=0.05 \times 35$

Using the law of conservation of momentum,

momentum of the rifle $+$ momentum of the bullet

$=4 \times \mathrm{v}-0.05 \times 35=0$

or $\quad v=\frac{0.05 \times 35}{4}=0.4375 \mathrm{~ms}^{-1}$