A 10 y/o plays football against the 49ers.
His center of gravity is a point in his torso. The 49ers COG is right in front of the QB, most of the time, although it can move around a bit.
On each play these COGs move toward each other and merge. They are separated, and on the next play the COGs merge again. The 10 y/o COG moves toward the 49ers and the 49ers toward the 10 y/o, and they merge.
Asked about it after the game, the 10 y/o, with no formal training in cosmology, physics, astronomy, or allied sciences, without even a complete 5th grade education, replied, "It sucked!"