This Demonstration determines the center of mass of a two-dimensional system of four particles. Use the sliders to change the masses of the particles. Check/uncheck the "show table" box to show/hide the masses and coordinates of the particles. Check the "show center of mass" box to show the coordinates of the center of mass in the table. For a two-particle system, set and to zero; for a three-particle system, set or to zero. Drag to change the coordinates of each particle and the position of the table.