In the figure shown below, all the contact surfaces are perfectly smooth. “M” and “m” indicate mass. When the object of mass “m” starts sliding down due to the force of gravity, then the centre of mass will _____

The center of mass of a distribution of mass in space is the unique point at any given time where the weighted relative position of the distributed mass sums to zero. This is the point to which a force may be applied to cause a linear acceleration without an angular acceleration.