Hand Spinner: His Boundless Craze Gives Place To Reflections
Like most parents of young children, I was forced to notice the fashion of the "hand spinner". Although the FiveThirthyEight website said they were "overwhelmed", my kids brought two new ones home after a Saturday birthday, in addition to three hand spinners that were already theirs, plus two or three others Were lost at school.
Of course, bloggers have used the hand spinner as an opportunity to talk about science, such as Rhett Allain and his two articles on the calculation of rotation time and measurement of the moment of inertia of the hand spinner. I usually managed to stay away from this trend, until I read an article from the Mashable site about the evolution of the hand spinner in space, which led me to express my point of view on Twitter . Having officially over-reflected on the subject, I would do as well to post a blog post on it ...
The question posed by Mashable is whether a hand spinner would rotate forever in space. The final answer is no, but like most questions involving physics, it ultimately depends on the definition of "always". This is how the test is performed - as I noted on Twitter, the big question is whether someone holds the spinner, or if it has been dropped and is in free evolution in space . If the astronaut who is doing the test holds the spinner in his hand, I think the object will stop in a time more or less equal to that needed on Earth, but if it floats freely, it should rotate to a long moment.
Explaining this effect requires some reflection on what, first of all, causes the spinner to stop. The key problem is the "angular momentum", the physical quantity associated with the rotational motion. Noether's theorem tells us that this is a consequence of rotational symmetry in the laws of physics, which means that the entire angular momentum of the universe should not change with time. Thus, when a thing begins to twirl, this kinetic moment can not spontaneously disappear: it must be transferred to something else.
In the case of a spinner on land, this transfer occurs because the spinner must generally be held against gravity, which means that it is in contact with something much bigger than itself - the hand of The person holding it, or a surface on which it is laid. This has the effect of keeping the central ring of the spinner still, with the outer parts spinning according to that larger thing, on a very good ball bearing. As effective as this bearing is, however, there is friction between the spinner and the central ring. This leads slowly to stopping the spinning - in terms of angular momentum, the friction of the bearing causes a twisting moment on the rotating part, which transfers the angular momentum of the initial spin to what holds it. On Earth, it is the person who holds the spinner in his hand, or the table on which it is placed, which in turn is placed on the ground; In the end, the rotation of the entire Earth increases or decreases infinitesimally when the spinner stops.
If the spinner was held by an astronaut floating in space, I would expect pretty much the same result: after about the same amount of time it takes for a hand spinner on Earth to stop, the moment Kinetics of the original spin would be transferred to the astronaut. In principle, this should cause the astronaut to rotate in flotation, but since the mass of an astronaut is about a thousand times that of a spinner, and its size between 10 and 100 times larger, it would turn around at about One-millionth of the initial spin rate, which would not be noticed.
The most interesting case would be that of an astronaut initiating the spinning of the spinner, then releasing it, so that it floats freely. In this case, there is always friction in the bearing, but this time the central ring is not anchored. So the friction can only equalize the turning rate of the outer spinner and the inner ring of the bearing. This will produce a small speed reduction for the spinner, and a large increase in speed for the central ring, until the entire object evolves uniformly.
At this point, another force is needed to transfer the angular momentum of the spinning spinner to something else. If one is inside a spatial habitat type such as the International Space Station (ISS), the obvious source of such a force would be the air resistance, which would slowly transfer The angular momentum of the spinner to the air molecules in the SSI (and then to the SSI itself). I'm not sure how long it will.
Of course, bloggers have used the hand spinner as an opportunity to talk about science, such as Rhett Allain and his two articles on the calculation of rotation time and measurement of the moment of inertia of the hand spinner. I usually managed to stay away from this trend, until I read an article from the Mashable site about the evolution of the hand spinner in space, which led me to express my point of view on Twitter . Having officially over-reflected on the subject, I would do as well to post a blog post on it ...
The question posed by Mashable is whether a hand spinner would rotate forever in space. The final answer is no, but like most questions involving physics, it ultimately depends on the definition of "always". This is how the test is performed - as I noted on Twitter, the big question is whether someone holds the spinner, or if it has been dropped and is in free evolution in space . If the astronaut who is doing the test holds the spinner in his hand, I think the object will stop in a time more or less equal to that needed on Earth, but if it floats freely, it should rotate to a long moment.
Explaining this effect requires some reflection on what, first of all, causes the spinner to stop. The key problem is the "angular momentum", the physical quantity associated with the rotational motion. Noether's theorem tells us that this is a consequence of rotational symmetry in the laws of physics, which means that the entire angular momentum of the universe should not change with time. Thus, when a thing begins to twirl, this kinetic moment can not spontaneously disappear: it must be transferred to something else.
In the case of a spinner on land, this transfer occurs because the spinner must generally be held against gravity, which means that it is in contact with something much bigger than itself - the hand of The person holding it, or a surface on which it is laid. This has the effect of keeping the central ring of the spinner still, with the outer parts spinning according to that larger thing, on a very good ball bearing. As effective as this bearing is, however, there is friction between the spinner and the central ring. This leads slowly to stopping the spinning - in terms of angular momentum, the friction of the bearing causes a twisting moment on the rotating part, which transfers the angular momentum of the initial spin to what holds it. On Earth, it is the person who holds the spinner in his hand, or the table on which it is placed, which in turn is placed on the ground; In the end, the rotation of the entire Earth increases or decreases infinitesimally when the spinner stops.
If the spinner was held by an astronaut floating in space, I would expect pretty much the same result: after about the same amount of time it takes for a hand spinner on Earth to stop, the moment Kinetics of the original spin would be transferred to the astronaut. In principle, this should cause the astronaut to rotate in flotation, but since the mass of an astronaut is about a thousand times that of a spinner, and its size between 10 and 100 times larger, it would turn around at about One-millionth of the initial spin rate, which would not be noticed.
The most interesting case would be that of an astronaut initiating the spinning of the spinner, then releasing it, so that it floats freely. In this case, there is always friction in the bearing, but this time the central ring is not anchored. So the friction can only equalize the turning rate of the outer spinner and the inner ring of the bearing. This will produce a small speed reduction for the spinner, and a large increase in speed for the central ring, until the entire object evolves uniformly.
At this point, another force is needed to transfer the angular momentum of the spinning spinner to something else. If one is inside a spatial habitat type such as the International Space Station (ISS), the obvious source of such a force would be the air resistance, which would slowly transfer The angular momentum of the spinner to the air molecules in the SSI (and then to the SSI itself). I'm not sure how long it will.
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