And now it’s all this

I saw Steve Mould’s latest video this morning. It’s about mechanisms that store elastic energy and release it quickly. His examples come from insects and human-made devices. You should take 15 minutes and watch it.

In previous reviews of Mould’s videos (here and here), I did some mechanics analysis to explain the videos in a more detailed (or just plain different) way. I’m not going to do that today, but I do have a brief comment on the question he raises at the end of the video, starting at about the 11:30 mark.

It gets harder and harder to open this thing up as the elastic becomes more and more stretched. But actually, beyond a certain point it gets easier again. And that’s because of the shallow angle of the elastic. Only a small component of the force from the elastic is perpendicular to the arm. So it gets easier and easier and easier until it passes the pivot point. It’s actually slightly holding it open now. And you only need a really small force to close it again.

Mould felt sure he’d seen this kind of mechanism before, but he couldn’t come up with an example. He asked around and didn’t get any satisfactory answers, probably because he was asking a bunch of physicists and mathematicians.

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