** What am I seeing? How does this work? **
The length of time it takes a ball to swing back and forth one time to return to its starting position is dependent on the length of the pendulum, not the mass of the ball. A longer pendulum will take longer to complete one cycle than a shorter pendulum. The lengths of the pendula in this demonstration are all different and were calculated so that in about 2:40, the balls all return to the same position at the same time – in that 2:40, the longest pendulum (in front) will oscillate (or go back and forth) 50 times, the next will oscillate 51 times, and on to the last of the 16 pendula which will oscillate 65 times. Try counting how many times the ball in front swings back and forth in the time it takes the balls to line up again, and then count how many times the ball in back swings back and forth in the same time (though it's much harder to keep your eye on the ball in back!).

** Why are they not perfect at the end? **
This large frame is built from wood and is outdoors, which means it expands, contracts, and flexes. Because the position of the frame changes, the cycle lengths are not perfectly aligned. Variable energy loss due to air friction and the striking of the pipe at the bottom (which creates the music) also contribute to differences. Over time, the minor differences become more pronounced.

Credit: Maria Ikenberry

Source: https://youtu.be/YhMiuzyU1ag