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

$A$ 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:

$Assertion$ : If no external force acts on a system of particles,then the centre of mass will not move in any direction.
$Reason$ : If net external force is zero,then the linear momentum of the system changes.

Two spheres of masses $2M$ and $M$ are initially at rest at a distance $R$ apart. Due to mutual force of attraction,they approach each other. When they are at a separation $R/2$,the acceleration of the centre of mass of the system would be ....... $m/s^2$.

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

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