When Nikola Tesla said, “The progressive development of man is vitally dependent on invention. It is the most important product of his creative brain.”, he didn’t account for foot massagers, electronic egg boilers (yes, that’s a thing now), or the newest fad — fidget spinners.
But fidget spinners aren’t just flamboyant toys. Fidget spinners, as the name suggests, were originally intended to help fidgeters or people who have trouble focusing by relieving their nervous energy.
Before understanding its behavior in space, let us first explore what concepts govern its motion.
A fidget spinner can be made of plastic or steel. It comprises two or (the usually preferred) three lobes, which when pushed, force it to rotate around the central axis. At the center are two rings that form two concentric circles. Between them are finely spaced ball bearings on which the outer ring rolls.
This region is where the magic occurs. Ball bearings reduce the the friction between the two rings and allow a spinner to rotate for longer amounts of time. This makes sense because shoving one slab over another would greatly increase friction and reduce the time that one ring might rotate over another — sliding friction is typically greater than rolling friction. Its spin time can be further increased by making the edges heavier, but why is that?
Angular Momentum and Force
Rotational kinematics is the branch of physics that governs the motion of a spinner. For starters, the concept of rotational kinematics can be viewed as the rotational or angular counterparts of linear kinematics. One of them is angular momentum, which is the angular component of linear momentum. Momentum is defined as the quantity or impetus of motion.
Angular components of linear motion can be calculated by performing a cross-product of linear quantities with the radius of the circle that the motion would draw. There is more on these technical details here. If the linear momentum of a body in motion is the product of its mass and linear velocity, then by the above logic, angular momentum or the quantity of rotational motion is proportional to mass, velocity and radius.
Therefore, an object in linear or rotational motion possessing a large amount of momentum tends to stay in motion for longer times. Now that we know that spin time is a function of momentum, we can infer why the lump of excess mass is placed farthest from the center — its radius.
The spinner derives its angular momentum from the push you provide it. Rotational motion arises from rotational force known as a torque. Similar to its linear component, it is the rate of change of momentum (angular here).
Torque is collectively generated by two parallel forces acting in the opposite direction. Consider opening a bottle cap, which requires you to pull one side with your thumb and push the opposite side with your index finger, just like pushing a spinner’s edge.
Why do rotating objects stop rotating?
Now that you’re familiar with all the terms, I can come to the crux of the article: For no external torques, the change in angular momentum is zero or the momentum is a constant.
This line of thought can be inferred from Newton’s First Law of Motion, which insists that in the absence of external forces, the momentum (linear or rotational) of a system will always be conserved. The law of conservation implies that momentum cannot simply be ‘lost’, but must be converted or transferred to something else.
Momentum lost somewhere is gained elsewhere. Consider an ice skater who increases her momentum to spin faster by folding her hands and concentrating her mass by reducing the radius. However, friction between her skates and the ice won’t allow this to go on for too long.
The spinner – or for that matter, any rotating object – comes to a halt by transferring its angular momentum to either directly connected objects or by dissipating its kinetic energy as heat and vibration into the environment.
How long will a fidget spinner rotate in space?
Now, let’s finally take our spinner into space and give it a whirl.
After supplying it with a finite torque, the spinner can achieve stagnation in two ways:
First, the spinner can be rotated between an astronaut’s finger or anchored to something, this makes for a fixed axis. Technically, the spinner will come to halt by transferring its momentum to the astronaut or the object it is anchored to. In principle, this would cause a free-floating astronaut to rotate!
However, since the mass of an astronaut is much larger than a spinner and the radius is now 10-100 times larger, the rate of spin is one-millionth of the original, which you’d obviously never notice. Thus, it would eventually stop.
Another way of rotating it would be to push a lobe and let it float, hanging in space. In the absence of any fixed axis and external forces (remember, there is no air resistance offered in space other than space dust, which we’ll assume isn’t in the vicinity for this proposed experiment), where would the momentum be transferred? Nowhere!
Although friction between the ball bearings would help dissipate the kinetic energy as heat into the surroundings, which would finally terminate its rotation, calculations show that this might take around a few billion years. The sun would expand and engulf it – along with us – before the spinner ever came to a halt!
If you forget friction for the time being, the spinner would practically rotate forever!
- Massachusetts Institute of Technology
- Isaac Physics; University of Cambridge
- University of Central Florida
- University of California Santa Cruz