Physics - Center of Mass Question with Solution | TestHub

For a system of particles, the motion of the center of mass is governed by the net external force acting on the system: ${\vec{F}}_{ext}=M{\vec{a}}_{CM}$.

When two particles interact only through mutual gravitational attraction, these forces are internal to the two-particle system. There are no other external forces mentioned.

Since the net external force on the system is zero, the acceleration of the center of mass (${\vec{a}}_{CM}$) is zero. This implies that the velocity of the center of mass (${\vec{v}}_{CM}$) remains constant. If the center of mass was initially at rest, it will remain stationary. Both A and R are true, and R correctly explains A.