Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Please Help Why does gravity make things spin?

  1. Mar 28, 2007 #1
    Please Help!! Why does gravity make things spin?


    I'm just getting into astronomy. I have tried to google this question many times but I cannot find an answer. I need to know why gravity makes things spin. I can understand the description of looking at space like a flat membrane. Things with mass, like planets, are heavy. And if you put one on this flat membrane it sinks down and bends the membrane down a little.
    If something gets close enough to this depression, it will slide down this slope, and collide. Essentially gravity.

    So I can understand this, gravity pulls things together, because the heavier object will have a deeper depression in space, and lighter objects will slide twoard it.

    But this doesnt explain why it makes things spin. How does gravity make our planet spin on its axis? How does gravity make the planets spin around the sun? Why does the moon spin around the earth and not just collide with it?

    It doesnt make sense!! Someone help please!
  2. jcsd
  3. Mar 28, 2007 #2


    User Avatar
    Science Advisor

    Try reading http://en.wikipedia.org/wiki/Orbit" [Broken] Wikipedia entry on orbits. The very short answer is that it is not some magical property of gravity per se that causes many things in nature to rotate, spin or orbit but just basic Newtonian physics. Have a read of that entry then come back with any more specific questions you have.

    Good luck! :smile:
    Last edited by a moderator: May 2, 2017
  4. Mar 28, 2007 #3
    Ok, I have read your website. From there I under stand how objects orbit. By looking at this picture, I can understand how an object fired out of this cannon may or may not orbit depending on how fast it is fired. If it is fired with a low velocity, it will end up at point A. With a little more velocity it will end up at point B, and with too much, it will escape the earths gravity, and continue out in space as in point E. You must have the perfect velocity to keep something in a circular orbit.

    This may explain why the moon stays in orbit around the earth, and how the planets stay in orbit around the sun.

    1) But it doesnt explain why the earth spins on its axis. Also,

    2) It doesnt explain what caused objects to move at this perfect velocity the first place. I'm pictureing a huge cloud of dust. I can see that all over the cloud, pieces clumping together. Eventually, large clumps would form spread through the cloud. Nothing would be in a perfect disk. And eventually, the large clumps would suck in everything that was close enough, and you would just have large clumps of matter, spread at large distances from each other, all through out this cloud. They would be sitting still, not close enough to any of the other large clumps to effect each other.

    So in order for our solar system to form. Gravity would have to get these large clumps moving again.

    Can someone explain 1 and 2?

    Attached Files:

  5. Mar 29, 2007 #4


    User Avatar

    Staff: Mentor

    The earth, moon, and planets formed from clouds of gas and dust that were non-uniform and randomly moving. When they collapsed, these motions and non-uniformities induced rotations. Only a perfectly symmetrical, perfectly still cloud of debris could collapse into a non-rotating object.
  6. Mar 29, 2007 #5
    Ok, If they are all randomly moving, why in the end do they all move the same direction?

    Would it make sense to have some of our planets orbit one way, and others in a different way?
  7. Mar 29, 2007 #6


    User Avatar
    Science Advisor

    Try http://en.wikipedia.org/wiki/Solar-system#Formation" wiki page on solar system formation. The key concept here is angular momentum.

    Sorry to be handballing your questions to a website, but since your question is so fundamental it requires a somewhat lengthier explanation than a bulletin board post would allow. The wiki page explains this pretty well I think, but again anything your not sure of after reading that post away :smile:
    Last edited by a moderator: Apr 22, 2017
  8. Mar 29, 2007 #7


    User Avatar

    Staff: Mentor

    Add all that random motion together and there is a net motion in one direction.
  9. Mar 30, 2007 #8


    User Avatar
    Science Advisor
    Gold Member

    Affirming Wallace and Russ, it's all about conservation of angular momentum. Consider what happens as a star forms from a collapsing gas cloud [forget about planets for now]. Lets also assume the initial gas cloud is not 'rotating'. As the particles are drawn toward the center of gravity, they collide. This deflects their course away from the perfectly straight line they were following toward the center of gravity. If they acquire insufficient velocity to escape the gravitational well of the gas cloud, what path do they take? - a spiral, of course. As the gas cloud densifies, the spirals flatten out. It gets increasingly difficult to approach the center of gravity because collisions become more frequent, hence the paths grow increasingly circular. Please note it is virtually impossible for all the collisions to cancel out. While at first the paths will be purely random, the tiniest of imbalances [like the gravity of the nearest star] will impart a preferred direction of travel.
  10. Mar 30, 2007 #9
    I like Chronos's explination. Conservation of angular momentum is THE main factor. A byproduct of inertia and velocity. You could also refer to the classic example of sitting in a spinning chair with your arms extended, then pulling your arms inwards to your chest... your rotation speed increases.

    A good example of this is the moon slowly drifting away from the Earth. The moon causes fluctuations of the tides which, by friction, is slightly causing the Earths rotation to slow. That rotation slowdown causes the moon to increase in distance... where an increase in rotational speed would cause the moon to drift closer.

    Think of the moon as your arms in that example. Another good one would be using a star as an example. Say you have an red giant star that is slowly rotating. When that giant runs out of fuel and can no longer push outward to resist the pull of gravity... it collapses in to a fast spinning white dwarf.
  11. Nov 21, 2007 #10
    My astronomy professor taught me that the tendency of bodies to spin in space is an unexplained phenomenon, and he said he attended an exhibit about this at NASA's Glenn Research Center. The links recommended in this forum sidestep the question being asked here, and only Chronos has truly offered an explanation. The conservation of angular momentum does not, however, explain why a pencil will suddenly start to spin if an astronaut lets it float inside a spacecraft without applying any force on it. Furthermore, it does not explain why the spinning motion is uniformly counterclockwise. There seems to be a larger impetus at work, but I don't have any guess as to what it might be. I feel your frustration, Tarnix: I haven't been able to find anything on Google about it either. You'd think there would be more theories out there.
  12. Nov 21, 2007 #11


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    This spontaneous spinning you speak of is news to me. Please link to a verifiable source.
  13. Nov 21, 2007 #12
    That's interesting. I wonder if anybody could tell me: Would a perfect ball start spinning as well in this situation? Would this happen even in a cockpit without atmosphere?

    Also: Not everething is spinning in space. Space itself for example. And the largest structures are not spinning as I understand it. I do not know whether glaxy clusters spin around each other. I would doubt it as they seem to move away from each other. But the galaxies within the clusters themselves certainly spin - a motion which I would guess is due to their interaction through gravity? And would this galaxy spinning not mean that the motion was already there for any cloud that the solarsystem started out from? Also the idea that the cloud the solarsystem form from is just sitting there in space and then collapses i not right. It was very likely a violent neibourhood in the galaxy where this took place under influence of a very large star sending off very energetic material. And one more argument for the cloud being already in motion: The cloud would be put together from material which all vere in motion already: Gas, pulled in from intergalactic space to the galaxy and dust from past stellar explosions. This was no immobile cloud.

    Greetings from interested amateur.
  14. Nov 21, 2007 #13


    User Avatar

    Staff: Mentor

    If objects would start spontaneously spinning while in orbit, the entire space shuttle and all of our satellites would spin. Quite obviously, they do not.
  15. Nov 22, 2007 #14
    Some people have proposed a Homopolar motor mechanism to explain rotational motion of bodies in space with an electric current running through them. You are basically converting electrical current into rotational force. Currents run through the earth, and other objects in space, but i'm not sure if they persist for long enough to actually generate rotaional motion. That's a possibility, but i dont think it has been proved yet.

    Maybe meteors with a high angle impacts would give sufficient impulse to cause rotation. Their direction should cancel out on average, but that could be another factor.
    Last edited: Nov 22, 2007
  16. Nov 23, 2007 #15
    i might just be hijackin this thread here,but my physics teacher told me a cloud of dust came together and started rotating, and after some steps forms a star. he did mention 'no one knows why the dust cloud rotates' which are forbidden words id expect to hear from him cus he seemed all-knowing.anyone shed light on this?
    anyway tarnix my understanding of some of thems things you ask is centripetal force

    i cant find a decent article but its things like [tex] F= \frac {mv^2}{r} = \frac {Gmm}{r^2}[/tex]
  17. Nov 23, 2007 #16

    Chris Hillman

    User Avatar
    Science Advisor

    Well, what happens when a spinning figure skater pulls her arms in? By conservation of angular momentum, her spin rate increases dramatically! In the same way, when a dust cloud collapses, it will usually have some angular momentum at the start of the collapse (wrt "the center"), and the result is that the dust particles typically start to rotate dramatically about "the center" as the cloud collapses.
  18. Nov 23, 2007 #17
    "This spontaneous spinning you speak of is news to me. Please link to a verifiable source."

    Unfortunately, I cannot find one. I'm just going off of what my teacher told me, but he seems like a fairly reputable guy.

    "If objects would start spontaneously spinning while in orbit, the entire space shuttle and all of our satellites would spin. Quite obviously, they do not."

    I'll be the first to admit that I don't know what I'm talking about, but my guess is that the space shuttle is either already in motion which offsets the spinning, or a stationary spacecraft would have sufficient mass to be orbiting the Earth, which is essentially a large scale spinning of sorts. I guess what I'm referring to is when an object is not affected by gravity or any other force and just idly resting in space, supposedly it will spin. Perhaps I've been misled, but I tend to think that the topic has been brushed aside because sometimes people don't like admitting that they can't figure something out. I saw another forum about this topic and somebody explained it by saying something to the effect that: "I spin, you spin, we all spin: it's just a reference thing." I, however, would argue that it's a tangible, measurable motion that warrants an explanation - an explanation that could perhaps open up a new field of study.
  19. Nov 23, 2007 #18


    User Avatar
    Gold Member

    Well they sort of will.

    Orbiting objects, if left to their own devices, will (eventually) orient themselves so that their long axis points radially (tidal force). Of course, once there, they'll stop spinning, so it's sortta short-lived. And it takes along time.
    Last edited: Nov 23, 2007
  20. Nov 23, 2007 #19


    User Avatar
    Gold Member

    I think you're going to have to back this up.

    This sounds a lot like some rather specific circumstances being blown up, distorted and mangled into some sort of myth, like the toilet-flushing myth.
  21. Nov 24, 2007 #20

    Clockwise and counterclockwise in space would simply be a matter of viewpoint, wouldn't it? If you want to decide which way the Earth rotates you first have to make up your mind which way is up, isn't that right?
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook