1. According to physicsclassroom.com, periodic motion is defined as "a motion that is regular and repeating." But the example included does not factor in damping (it's assumed that there's no air resistance and the spring will keep vibrating for eternity.) http://www.physicsclassroom.com/class/waves/Lesson-0/Properties-of-Periodic-Motion 3. I know periodic motion means that the time it takes to complete each cycle is the same, so does that mean that in periodic motion there is never damping (so periodic motion is only a concept?) or can there still be damping where the amplitude decreases, eventually reaching 0...?