How long would a fidget spinner spin for in space?

04 September 2017

Question

How long would a fidget spinner spin for in space?

Answer

Chris Smith put this question to Jess Wade from Imperial College London...

Jess - I think Sean has to think about all the forces that are slowing the fidget spinner down once you start spinning it. And have you done any experiments at home where you started off spinning and see how long it can spin for and time it? Have the kind of audience of the Naked Scientists or have you tried it?

Chris - Oh yes. The kids have been actually doing that and actually, they're having competitions to see who’s got the best bearings in their fidget spinner.

Jess - And what kind of time can they get to?

Chris - Something like a minute and a half, 2 minutes.

Jess - So I think that’s kind of important; what you said before about bearings. So it’s working because of angular momentum and angular momentum is conserved. We don’t get rid of angular momentum. It’s just transferred from one thing to the other. So once you…

Chris - Angular momentum being…

Jess - Kind of, how things were – oh gosh, I'm really going to push this answer, but okay, you can imagine it really nicely when you think about a ballerina or an ice skater spinning around and the person starts to spin around and puts out their arms out when they start spinning around and they spin around really, really slowly. And as they bring their arms in, they start to spin around much, much faster, and that’s because angular momentum is conserved and it’s actually, that the moment of (13:28) the thing stopping it spinning around is much, much smaller when you’ve got your arms inside. So, it’s kind of this motion in rotational direction.

Chris - So, instead of moving a big mass through a big circle, you move the same mass through a much smaller circle…

Jess - Exactly, right.

Chris - …so therefore, you have to go faster to keep everything balanced.

Jess - To keep everything balanced.

Chris - So that’s why your ice skater or ballerina speeds up. And the fidget spinner has got this property angular momentum and it can't get rid of it. It can't stop spinning.

Jess - It can't stop spinning and actually, the way that it stops spinning is because it transfers a tiny bit of that angular momentum to the earth. So as it’s spinning around, it makes the earth spin around the other way a tiny, tiny bit slower. And it starts to slow down over time. The other thing that you have happening when you're spinning your nice little fidget spinner in here is you have air resistance. So you have something pushing it back. You have something that is going up against, and you obviously have this friction of this ball bearing. So lots of things are going against your nice fidget spinner, making it slow down in about 2 minutes. If you take it up to the ISS and I'm sure they’ll take one up to the ISS the next time they go up.

Chris - International Space Station.

Jess - Yes, sorry. International Space Station, friends, biology friends, but you probably knew that. But if you took one up to the ISS, you're taking, you're reducing that force of gravity. So you’ve really only got the air resistance. So you’ve got air resistance going against you and it would probably still take – I mean, you could do it in probably years, I’d say, if you were reducing the air resistance. If you put it on the outside of the ISS, you're going to a much lower pressure that’s much less air resistance out there. I would say, kind of ‘timesing’ in – what's the pressure on the outside of the ISS? About 10 trillionth of the pressure in here, so probably…

Chris - It is probably 1 atom per cubic meter I think is the accepted density of space.

Jess - Not much. So, I would say it’s probably doing the maths. (15:10)we could ask a mathematician actually, but I would say tens of millions of years for it to slow down if it’s on the outside of the ISS.

Chris - It’s quite a long spin, isn’t it?

Jess - Yeah. So, there's an awful lot on earth that’s making that thing slow down and yet, even if you watch it, it still doesn’t seem to be slowing down very quickly. So, if you took that up to space, you'd have it for a lot longer.

Chris - I'm intrigued to think that by spinning my fidget spinner by making the earth spin the other way a little bit.

Jess - It’s great, right?

Chris - Because obviously, one has to think if enough people span their fidget spinner, would the earth slow down?

Jess - And I think you can also look. If you look on the Institute of Physics website, they have some great videos about that and also, the bottle flip. So, the bottle flip which was kind of the pre-fidget spinner nice demonstration of angular momentum and great kids trend is another thing that you can explore entirely through centre of mass and how different liquids that you put inside it, their viscosity and stuff like that. It’s a really fun experiment for kids today. You can film them in slow motion on your phone and you can watch the centre of mass stay entirely stationary as the bottle flips around. But that’s another great way that we can explore these things at home.

Chris - And if you actually want to have a look at how to do that experiment, there is a Kitchen Science experiment on our website explaining that one. You go to nakedscientists.com/kitchenscience.

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