Question

Two spheres of mass M and 2M float in space in the absence of external gravitational forces, as shown in the figure. Which of the following predictions is correct about the motion of the center of mass of the two-sphere system?

Answer 1

-- If the two spheres are both at rest when we arrive and start watching them, then their total momentum is zero.  

-- If their center of mass were to move, then that would mean that some momentum had somehow magically appeared.

-- But we know that momentum is conserved so this can't happen.

-- The center of mass must remain at rest. (A)