In a system of particles,internal forces can change (for the system)

Why do internal forces acting within a system not affect its motion?

$A$ rope ladder with a length $l$ carrying a man with a mass $m$ at its end is attached to the basket of a balloon with a mass $M$. The entire system is in equilibrium in the air. As the man climbs up to the end of the ladder into the balloon,the balloon descends by a height $h$. Then the potential energy of the man:

Internal forces acting within a system of particles can alter

If the total external forces on the system of particles is zero,then find the velocity and acceleration of its centre of mass.

Which types of forces act on a system of particles?