Slinky Physics

A nifty video showing the dropping of a Slinky. While that may not sound all that exciting, when seen in super slo-mo, you can see that the bottom end of the Slinky stays perfectly stationary while the Slinky drops, until the collapsing tops part reaches it. At first glance this may seem counter intuitive, but it actually makes perfect sense.

The bottom part of the Slinky – and indeed every part of it – is initially held stationary as the result of the balancing of the force of gravity pulling it down and the tension in the spring holding it it. As the Slinky is released at the top, there is no more tension force at the top, so it begins to react purely to the force of gravity. But the spring is still stretched out, exerting a tension force at every point along its length…  a tension force perfectly balanced by gravity. So as the top continues to fall, more and more of the spring is of course collapsed at the top end, removing the tension… but only for that length of the spring which has been collapsed. The balance of tension and gravity remains precise for all those portions of the Slinky where the spring has not collapsed.

[youtube eCMmmEEyOO0]

The interesting thing is that this same physics holds for *all* objects that are held at the top and then released. A Slinky is a good demonstrator because it is is so stretchy… but if you had a sufficiently fast and precise camera, you’d see the same result whether the suspended+dropped item was a Slinky, a baseball bat, a steel I-beam or a brick. In the case of the latter items, the effect would be far harder to see… they are not very stretchy at all, but every object made of normal matter does stretch at least a little bit when tension is applied.