Why do things spin?

Galaxies spin, tornadoes spin, electrons spin, water spins down the drain.

Why do so many things in our universe spin?

God made them that way.

First off, electrons do not actually spin. This is a term used to describe one characteristic of the electron. …just like subatomic particles really have no charm! It’s just a term…no different than describing an airplane’s attitude in space.

In general, anything with dimensions can move by translation (striaght-line) or rotation. Everything has dimensions, so everything may spin, if it is acted upon to set it into a spinning motion. Only points and point masses have no dimensions, but they are mere figments created for academic purposes.

When Newton said “a body in motion (or rest) will remain in motion (or rest) unless acted upon by an outside force”, he was saying that an object naturally follows a straight path unless some force acts upon it to cause otherwise. This was a major breakthrough at the time because Galileo was the lead authority performing many experiments and observations on motion like Newton, but Galileo assumed a curved path was the “natural” motion of a body - heavily influenced by his astronomical studies.

Hope this helps somewhat.

  • Jinx

Electrons do spin, well sorta. Electrons have two types of angular momentum, orbital angular momentum which is that peculiar as it arises from their orbital motion and spin angular momentum which is pretty danm peculiar and (sorta, cos it’s not really what’s happening) represents the electron spinning on it’s axis. These two quantities are added together in order to get the total angular momentum. Spin angular momentum is peculiar firstly because how can a point particle spin on it’s axis (as I said before this is not really what’s happening though)? and secondly for an electron it alaways has the same value (+ or - 1/2) (also spin in orthogonal directions are what as known as a complemntary pair in QM, i.e. measuring one causes uncertainity in the other, like momntum and position).

  • first sentance: isn’t that peculiar.

Don’t butterflies disprove this notion?

OK, any time I have a perplexing problem, instead of thinking about it, I can just plug in “because God made it that way”. My troubles are over. :rolleyes:

In classical (non-quantum) mechanics, central forces exert no torque, so angular momentum is conserved. A system that starts off with non-zero angular momentum will retain that same value of angular momentum in the absence of external torque. Hence your observation that galaxies and planets spin, since the external forces on them are central forces.

[ul]:dubious:[sup]** Only if you are a true believer.**[/sup][/ul]

Well hey, if it’s going to automatically solve all the mysteries of the world, sign me up. What’s it cost to join? :slight_smile:

If spinning puzzles you, think of it as “linear motion with an external force directed towards a point or centrum”. This si not a textbook physics answer, but it’s a pretty good way of thinking of i, and accurate in most cases. WARNING: gross simplifications follow.

An orbit is, crudely, the path dictated by an object’s (linear) inertia, acted on by a constantly changing grqvitational vector directed at the center of mass of the object it’s orbiting, This vector (a description of a force, including both its direction and magnitude) is changing because the orbiting object’s position is changing, so the direction, and often distance, to the other body is constantly changing. The object being orbited may also be moved by the object that is orbiting, soi it’s more accurate to say they orbit each other, even if one object is so much larger that its motion is negligible

In classical mechanics (which really doesn’t apply, but I don’t want to go all quantum on you), an electrons orbit is exactly the same but with the vector force being an electrostatic attraction. In a top, the centripetal force (force towards the center) is the tensile strength of the top - other wise it would explode in all directions, as each atom went flying off in a straight line (in the local direction of spin not radially outwards, though that’s how it’d look from a distance). In the case of a galaxy, the centripetal force is gravity again.

A hurricane, however is a far more complex case, and frankly I would not even try describing it. Local pressure, velocity, temperature, gas density, and the corresponding properties in all regions adjoining each sample space would all enter into it (along with many other factors, depending on your model). Basically it would require a sophisticated computer model or heavy duty calculus to even take a stab at balancing it all, and almost everyone who isn’t actually doing the modelling professionally relies on gross simplifications.

So, in the end, you get so-called circular motion (spinning - which isn’t always truly circular) when an outward force that is directed toward a point or centrum (a centrum is a mathematical point which may not be a physical object, like the center of an empty hollow ball - there;s nothing there, but it’s often useful to pretend all the mass is concentrated at this ‘weighted mathematical average’.)

I’m sure you can see that it’s really common for an object to seem to exert a force on another object from a distance [well, quantum mechanics may argue, but why get into exchange particles and fields here], so naturally ‘spinning’ is a common class of phenomena.

Does that answer your question int the terms you wanted?

Certainly the best answer I have seen so far (minus the typos), the most general anyway. :smiley:

I fear I’ve been swooshed, but what part of a butterfly’s flight can’t you describe with translation and rotation?

Yeah, I was real tired when I sent that response. I think anything living that has a way of locomotion has to be excluded. Let’s just think about non-living things.

I wonder if there are more unique things that spin in the natural world or the manufactured world? I would guess the manufactured world.

Now I’m really confused. For any moving body, regardless of what that body is, the path of motion can be broken down into translation and rotation. This has nothing to do with the OP about why things spin, but I’m curious about your reply to Jinx and why you think the superposition of translation and rotation doesn’t apply to living things.

As far as the OP is concerned, I think amore ac studio handed the case of galaxies pretty well. They spin because they started out spinning and nothing has stopped them. For other things like hurricanes, spin is caused by any slight imbalance in forces (e.g. the Coriolis effect that causes hurricanes to spin). It’s actually pretty hard to apply a force to something in just the right way to not cause it to spin. If water going down the drain has an momentum other than straight toward the drain, it will tend to circle the drain. If your car has any imbalance between the two drive wheels, it will tend to spin in circles instead of go straight and only a correction by steering prevents it. When you hit a golf ball, kick a soccer ball, or bat a baseball, the force almost always exerts some torque and causes spin and, even if it doesn’t, other forces like drag might induce a spin.

Well I didn’t want to get into a long discussion on how the forces mentioned apply to the locomotion of living things. They do indeed apply, but I wanted to keep it “simple” and just talk about natural phenomenon in our universe, other than living things.

It is simple. Things move in straight lines unless they have forces acting on them. Living things typically have forces acting on them. Therefore, living things do not necessarily move in straight lines.

The cost? Well, you have to have your brain spooned out, but it is a small price for peace of mind.

I wonder if there are more unique things that spin in the natural world or the manufactured world? I would guess the manufactured world.

That is a hopeful statement;
there are 2000 billion billion stars in the visible universe; all spinning.
Galaxies, stars and planets spin because they form when clouds of gas and dust collapse upon themselves due to their own gravity; these clouds are never completely motionless to begin with, and the varying amounts of momentum and much of the gravitational potential in the clouds are converted into a single rotating disk, out of which stars and planets form.

of course the number of spinning things in the manufactured world might well exceed the number of stars; I am hopeful that there are plenty of alien civilisations up there with spinning flywheels, cogs and yo-yos.