Which types of forces act on a system of particles?

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:

The multiplication of the total mass of a system and the acceleration of its centre of mass denotes what?

Two persons of mass $m_1$ and $m_2$ are standing at the two ends $A$ and $B$ respectively,of a trolley of mass $M$ as shown. When both the persons jump simultaneously with the same speed,then:

Assertion $(A):$ If there is no external torque on a body about its centre of mass,then the velocity of the centre of mass remains constant.
Reason $(R):$ The linear momentum of an isolated system remains constant.