Tales from the Prep Room: Magnus Cups

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Making spinning cups curve in unexpected ways.

In the wake of the Christmas Lectures, Ri Demo Technician Andy Marmery returns to an empty Faraday Theatre to test out the so-called Magnus Effect.

The Magnus Effect is a phenomenon observed when a spinning object moving through the air takes a curved, rather than straight, path.

First described in 1853 by German physicist Heinrich Gustav Magnus, the effect is common in ball sports when the ball is kicked and given a spin at the same time.

Here, the curved flight path is caused by a difference in pressure above and below the cup. The air on top is moving in the same direction as the cup, whereas the air underneath is moving in the opposite direction. The low pressure above the cup leads to the upwards motion.  

We got Andy to demonstrate the effect after DemoJam host Matt Parker promised we would show you how to make your own "Bernoulli" cups at home.

Many explanations of this and similar phenomena around flying things appeal to Bernoulli's principle, often incorrectly. Wikipedia even have a section covering "Misunderstandings about the generation of lift", which is worth a read. If you've a better explanation of what's happening here than Andy gives, we'd love to hear it - please leave a comment.


Engineering, Maths, Talking Science


Andrew Marmery
London, UK
Filmed in:
The Theatre, The Prep Room

Royal Institution 

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Tales from the Prep Room

cc_by-nc-sa License: Creative Commons



So today we're going to be making something which has been called the Bernoulli cups. Whether that's an appropriate name or not is something that I won't comment on. All we need for it are these two polystyrene cups that I'm going to arrange end to end somehow. And I'm just going to tape them together around the middle, like so. A little bit more, maybe.

Right, that should do the job, apart from the couple of elastic bands. These are probably not quite long enough, but we can do something about that. We'll just fold one through the other and make a loop, double-length elastic band. That's all we need. So let's go and find out what this does, for which we'll need a bit more room, I think.

OK, we're in the gallery of the lecture theatre, which obviously is the best place for launching flying objects, which hopefully is what this is about to become. Well, first of all, I have to just wind the elastic band around the middle, and hopefully wind it on itself so it doesn't pop out.

I wonder if I'm doing it the right way. I can't really remember what's the best way of doing it. And then I'll hold on to that a bit, hold it a bit like that. And, mm-hmm. Mmm, yeah, flying would be a generous interpretation for what just happened.

Oh, yeah, that's better. You sort of saw it loop up a bit before it ran out of steam and then kind of plummet. Well, luckily, I've got plenty more, so we can have a few more goes.

Ooh, a bit of a misfire.

So what I think we saw there was - well, first of all, the cups fell a lot more slowly than you might think they would. They drifted down pretty slowly. But also, I don't know if you saw at the start, when the cups were moving quite fast, they sort of curved up a bit before drifting back down again. So there's obviously some effect going on there.

Now, aerodynamics is pretty fiendish to explain. And getting two people to agree on the best way of explaining something, it's as likely to end in a fist-fight as not, I think. So I'm going to tread quite carefully. But one of the things that I think is going on here is something called the Magnus effect. And this applies to objects that are spinning as they move through the air. So it applies to footballs when - this is what makes footballs curve when the players apply spin to the ball.

So with the cups, as they move, they're sort of spinning backwards like this. And you get sort of air entrained around the cups, spinning with them. And that sort of creates a higher air pressure area underneath the cups, and sort of lower pressure on top.

And that supports some of the weight of the cup, even all of the weight of the cup at the start. And you get this sort of kicking up like that, and then just gradually slowly drifting down. And then eventually the spin runs out of oomph, and it just falls on the ground.

This is pretty easy to make, so have a go yourself. See what you think. And see if you can come up with a better explanation than I have given.

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