Law of conservation of linear momentum is derived by using Newton’s which laws ?

A mass $m$ moving horizontally (along the $x-$ axis) with velocity $v$ collides and sticks to a mass of $3m$ moving vertically upward (along the $y-$ axis) with velocity $2v$. The final velocity of the combination is

A body is moving with a velocity $v$, breaks up into two equal parts. One of the part retraces back with velocity $v$. Then the velocity of the other part is

A wagon weighing $1000\, kg$ is moving with a velocity $50\,km/h$ on smooth horizontal rails. A mass of $250 \,kg$ is dropped into it. The velocity with which it moves now is ......... $km/hour$

The momentum of a system is conserved

Two spheres $A$ and $B$ of masses $m_1$ and $m_2$ respectively collide. $A$ is at rest initially and $B$ is moving with velocity $v$ along $x-$ axis. After collision $B$  has a $\frac {v}{2}$ velocity in a direction perpendicular to the original direction. The mass $A$ moves after collision in the direction