How to Spot a Psychopath

A reader writes:

If you take a spring - a metal one I suppose (I know nothing about springs other than they're fun to play with) - and hang it by one end, with no weights or anything attached to the other end, and just leave it hanging there, will it eventually (like really eventually) become completely straight? Or what will happen? Does it matter what kind of spring it is? Will it's own weight straighten it out, or is there something about its structure that would prevent that from happening? And the follow-up, if the answer is yes, is, let's say it's a Slinky; about how long would it take?

I wish I could say I had a beer riding on this, but the truth is I'm just geeky and thought you might know.

Michael

No, a hanging spiral spring won't straighten.

The key concepts here are elasticity and plasticity. The whole idea of a spring is that it's elastic - you can stretch and/or compress it, and when you let go, it returns to its original shape. If the force applied to an elastic object exceeds the limits of its elasticity then the object will be permanently deformed (or just break), but you'd need a pretty darn long, but skinny, spring for that to happen just from the spring's own weight.

Slinkies are an extreme case, here, because they're a quite unusual kind of spring, with peculiar dimensions compared with most spiral springs. Dangling a brand new Slinky may actually give it a slight permanent stretch, but it clearly doesn't stretch it very much, and you can leave it hanging as long as you like without getting any more stretch than happens in the first few minutes.

It's actually normal for a new spring to distort somewhat when put to use. This is called "taking a set", and has to be accounted for in the design of devices that use springs, from the huge ones in heavy vehicle suspensions to the incredibly delicate ones in mechanical wristwatches. It's unusual for a spring to take a set just from its own weight, though.

If you made a spring out of, say, tin/lead electronics solder, then it wouldn't need to be very long in order to straighten out under its own weight. It'd probably continue to straighten for some time, too - meaning hours, though, not years. Tin/lead alloy is of course a terrible material for springs, since it's highly plastic and hardly elastic at all.

Apropos of this, there's a really neat guide to making your own springs here. Home handypersons usually regard spring-making as a black art and just end up with a parts box full of springs cannibalised from other items, but you really can make them yourself without being a master metalworker.

(Oh, and I know I sound like a broken record, but J.E. Gordon's "The New Science of Strong Materials, or Why You Don't Fall through the Floor" and "Structures, Or Why Things Don't Fall Down" both have a lot to say about the springiness of actual springs and of many other objects, and about the foundational concepts of stress and strain.)

Psycho Science is a regular feature here. Ask me your science questions, and I'll answer them. Probably.

And then commenters will, I hope, correct at least the most obvious flaws in my answer.