One of the things one learns studying acoustics (and many other physics topics) is that the behavior of a complicated physical system can often be\u00a0simplified\u00a0into an\u00a0analogy\u00a0of masses and springs. \u00a0The gobs of air that surround us have elasticity and they have mass, and these are the properties that allow waves to travel through the air as sound.<\/p>\n
Perhaps a more\u00a0intuitive\u00a0example of a spring-mass system can be found in any toy store: the classic Slinky<\/a>. \u00a0The familiar coil toy can be used to demonstrate lots of different wave phenomena (longitudinal waves, transverse waves, standing waves), and when that gets boring, it is more commonly used to\u00a0demonstrate\u00a0walking down stairs.<\/p>\n We recently came across this high-speed video of the very interesting spring-mass behavior of an extended Slinky at rest, dropped from height, in which the bottom end of the Slinky seems frozen in mid-air. \u00a0There are excellent technical explanations<\/a> of what is going on out there (and probably on a tricky physics midterm or two), but suffice it to say that it all goes back to the interplay between mass and elasticity as the Slinky simultaneously contracts and falls.<\/p>\n