Two balls in free space are colliding with each other. Which of the following statements regarding linear momentum conservation of the system is true?

$A$ block of mass $m$ is moving with a velocity $u$ on a smooth horizontal surface towards a wedge of same mass $m$ initially kept at rest. The wedge is free to move in any direction. Initially, the block moves up the smooth inclined plane of the wedge to a height $h$ and then moves down back to the horizontal plane. In this process, the wedge gains a velocity equal to:

You are on a frictionless horizontal plane. How can you get off if no horizontal force is exerted by pushing against the surface?

The principle of conservation of linear momentum can be strictly applied during a collision between two particles provided the time of impact is

If the resultant of all the external forces acting on a system of particles is zero,then from an inertial frame,one can surely say that

Three masses $m$,$2m$,and $3m$ are moving in the $x-y$ plane with speeds $3u$,$2u$,and $u$ respectively,as shown in the figure. The three masses collide at the same point $P$ and stick together. The velocity of the resulting mass will be