The Science of Slinky Drops
Watching QI is, without fail, a great way to learn new things that make me go… “Hah that’s quite interesting!” (yeah, you saw that coming!). This time Stephen Fry was talking about slinky drops, specifically how the bottom of the slinky does not move when it is released from the top.
How does this happen? Unsurprisingly, gravity is involved. When we drop an object, we remove the force supporting it that balances the weight of that object, so that it accelerates towards to ground due to gravity. So far so good. But there are additional forces at play when it comes to a slinky.
The hanging slinky is ‘stretched out’, or under tension, because of its own weight. The slinky is in equilibrium, so the tension equals the weight. When released, the tension changes, disrupting the equilibrium. This change is propagated through each coil of the slinky.
The bottom of the slinky is the last part to experience this change and remains in equilibrium between a downward force due to weight and an upward force due to tension. This is until the change in tension propagates to the last coil. The bottom of the slinky experiences no net forces (equal weight and tension) before, and immediately after release. We know from Newton’s First Law of motion that an object at rest stays at rest unless acted upon by an external force. So in the case of the slinky, that is exactly why the bottom of the slinky does not move.