$A$ gun recoils after firing to conserve

Two friends on roller-skates are standing $5\, m$ apart facing each other. One of them throws a ball of $2\, kg$ towards the other,who catches it. How will this activity affect the position of the two? Explain your answer.

State Newton's third law of motion. Give two examples.

$A$ rocket works on the principle of conservation of

$A$ bullet of mass $m$ is fired from a gun of mass $M$. If the velocity of the bullet is $v$,derive an expression for the recoil velocity of the gun. On which principle is this based?

$(a)$ Define momentum. Write its $SI$ unit.
$(b)$ $A$ bullet of mass $0.02 \ kg$ is fired from a gun weighing $7.5 \ kg$. If the initial velocity of the bullet is $200 \ m \ s^{-1}$,calculate the speed with which the gun recoils.