A rifle man, who together with his rifle has a mass of 100 kg, stands on a smooth surface fires 10 shots horizontally. Each bullet has a mass 10 gm and muzzle velocity of 800 m/s. What velocity does rifle man acquire at the end of 10 shots in ms^-1

Let m ₁and m₂ be the masses of bullet and the rifleman and v₁ and  v₂ their respective velocities after the first shot. Initially the rifleman and bullet are at rest, therefore initial momentum of system =0 As external force is zero, momentum of system is constant i.e initial momentum = final momentum
$\begin{array}{l}0={\mathrm{m}}_1{\mathrm{v}}_1+{\mathrm{m}}_2+{\mathrm{v}}_2\\ {\mathrm{v}}_2=-\frac{{\mathrm{m}}_1{\mathrm{v}}_1}{{\mathrm{m}}_2}=-\frac{10\left(10\times {10}^{-3}\mathrm{kg}\right)(800\mathrm{m}/\mathrm{s})}{100\mathrm{kg}}\\ =-0.8\mathrm{m}/\mathrm{s}\end{array}$