objects x and y are connected by a string of negligible mass and suspended vertically over a pulley of negligible mass, creating an atwood’s machine, as shown in the figure. the objects are initially at rest, and the mass of object y is greater than the mass of object x. as object y falls, how does the kinetic energy of the center of mass of the two-object system change? justify your selection. all frictional forces are considered to be negligible.