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

$A$ 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:

Under which force the centre of mass is moving?

If a force $10 \hat{i} + 15 \hat{j} + 25 \hat{k}$ acts on a system and gives an acceleration $2 \hat{i} + 3 \hat{j} - 5 \hat{k}$ to the centre of mass of the system,the mass of the system is

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.

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