1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

What is the motion of spin relative to?

  1. Jan 5, 2009 #1
    If all motion is relative, what is the motion of spin relative to? Whether a thing is spinning or not is evident by the way the thing behaves. A spinning top will “stand up” but a non-spinning one won’t. As far as I understand it, the properties of the thing spinning are totally independent of any other thing or motion. This seems true at any scale from a top, planet, solar system or galaxy.
  2. jcsd
  3. Jan 5, 2009 #2
    Someone might correct me but here's my take, and I'm pretty sure on this:

    The motion is relative to whatever you want. So the motion of a spinning top is relative to the "stationary" table it sits on, you as an observer, the Earth, and everything else, based on each things particular velocity. The relativity of the motion isn't the issue that causes a spinning top to "stand up", it's the kinetic energy of the spin. The energy invested in spinning exists independently to it's relative velocity. So just because you can treat the top as stationary and the table as spinning under it for mathematical purposes, the energy of the spin still exists. The reason a top stands up when you spin it is a combination of the effect of centripetal force from the angular energy of the spin and the Earth's gravity keeping the table and the top in contact (thus negating most of the vertical momentum). The angular momentum is independent of the relative velocities of other bodies, but the energy does still exist.

    So let's say that in a vacuum, you were observing a spinning top, spinning at a rate of "x". It wouldn't matter if you treated the top as spinning or you treated yourself as an object orbiting the top at the speed of "x". If you did start from the perspective that the top was spinning, and then accelerated in orbit of the top to the rate of "x" then there would be no difference between you and the top relative to each other. You would both appear stationary from the others perspective, and could be treated as stationary for most purposes. You could reach out and touch a single point on the top and it would not appear to be moving.

    However, the energy is still there. You are, in fact, still moving relative to everything else in the universe. Either one of you can stop without affecting the movement of the other. If, for instance, while orbiting the top you ran into a steel wall with a relative velocity of zero to you and the top, you would notice quite quickly that you and the top did, in fact, possess kinetic energy. :) For mathematical purposes of course this is no different from the wall being accelerated to x while you were stationary, but it would come as little consolation in whatever afterlife you believe in.

    Note that you can tell that energy is independent of relative velocity because it doesn't affect two objects identically, depending on their mass. For example, if an astronaut were suspended in a zero gravity vacuum and spun a top, the astronaut would not begin spinning at the same rate as the top relative to another observer. When he spins the top he gets the same amount of momentum the top does in reverse (Newton's third law), but being a much larger mass he likely would barely notice it.
    Last edited: Jan 5, 2009
  4. Jan 6, 2009 #3
    The motion is simply relative to the axis of spinning, and axis ONLY. In addition to mass, everything in the universe has a moment of inertia. Just as mass the moment of inertia is also independent of any other thing. That is the only independent property. Everything else is relative, the act of spinning, and even angular momentum, rotational energy, etc!

    Just like linear motion is relative, you cannot tell if something is spinning or not just by
    looking at it. You need other means to detect. Only thing you can say is that the object you are looking at "appears" to be spinning on its axis. Thats it. You cannot tell if it is indeed spinning of you are revolving around it the other way :-D.

    If a stationary top is suspended in air, and the table rotates around the axis and you are sitting on the table and looking at the top, the top will appear to be spinning "relative to you" the other way. The top will have angular momentum too, AS MEASURED BY YOU, that is.

    If the top is a star and the table and you are planet, from the planet's point of view you cannot tell if you are revolving around it or the star is spinning the other way, JUST BY LOOKING AT IT! Of course you will have psuedo force that tells you if you are revolving or if the star is spinning....
  5. Jan 6, 2009 #4


    User Avatar
    Homework Helper
    Education Advisor
    Gold Member

    What about the subatomic level; does the direction of spin axis mean anything? There are "up" and "down" spins, or there are plus half and minus half spin numbers, but does direction of axis mean anything for that level?
  6. Jan 6, 2009 #5

    That doesn't seem right. If it was me that was revolving around it, I would in fact be experiencing an acceleration. This is not an inertial frame we are talking about and so it must be treated differently.
  7. Jan 6, 2009 #6
    This is the famous "Newton`s rotating bucket paradox" - not yet satisfactorily explained, as I understand.

    A bucket containing water is rotating generating a parabola shaped surface. Would that
    parabola shape occur even if it instead was the world around the bucket that rotated? - and aquainted questions.

    So the question is: "What is the true reference for the rotation, from which centrifugal force can be calculated ackording to classical formula?". Hitherto scientists have answered: "Angular velocity related to distant fixed stars", although admitting this is not a fully satisfying answer.

    Personally I think it is just a matter of how the rotation is related to free particles
    movements left alone at the place of rotation. :rolleyes:
    Last edited: Jan 7, 2009
  8. Jan 7, 2009 #7


    Staff: Mentor

    The usual meaning of the phrase "X is relative" is that different inertial reference frames will disagree on X. Velocity is relative, as are durations, distances, energy, momentum, frequency, wavelength, etc. Note, energy is relative, previous posts to the contrary are incprrect: it has the same relative relationship to momentum that time durations have to spatial distances.

    Things which are not relative include proper time, proper distance, proper acceleration, phase, mass, etc. Rotation is a type of proper acceleration so it is not relative and all inertial frames agree.
    Last edited: Jan 7, 2009
  9. Jan 7, 2009 #8
    Thanks for the replies. I haven't had time to post but I have been reading.
  10. Jan 7, 2009 #9
    A thing that is spinning will experience the properties of spinning regardless of how any other thing is moving relative to it or how it is observed. What is the stationary position that causes a thing to be spinning or not? “Not spinning” is a description not an explanation. It seems to me that the universe has some form of common stationary orientation.
  11. Mar 27, 2011 #10
    Hello all,
    This thought experiment has fascinated me for (a lot) of years. My apologies if the following thoughts are not helpful, but I would love to know some answers.
    a) Firstly, consider a universe that consists solely of a big, massive, spinning top (say a star) and one hapless astronaut who is orbiting it. How can the astronaut tell whether the star is spinning? Allowing considerable freedom from conventional reality, the astronaut could reach out (his arm is long enough) and try to turn the star. If the star is spinning, he will have to use more effort to change the angle of the star (due to its angular momentum I think) if it is spinning than if it is not spinning.
    Given that the universe in this thought experiment consists solely of the star and the astronaut, what is the star spinning relative to?
    b) Taking Newton’s “bucket” idea as a startpoint, raises some other odd thoughts. For a start, forget the rope between them, gravity will do. Any such universe would have very limited options:-
    It could collapse very quickly due to gravitation.
    It could continue to expand because both bodies were already moving apart (big bang).
    It could remain stable, possibly for a long time, if the two bodies were rotating, but what would they be rotating relative to? As these are the only two objects in this pretend universe, it would imply a rotating universe and then a frame of reference against which that rotation would be operating – or that the two bodies had an intrinsic property that we do not normally express?
    c) Along with everything else that I have asked, I find the concept of a rotating universe hard to understand. In an expanding universe, there would be a limit on the velocity of the outermost stars – otherwise they would eventually break relativistic laws ( they could not exceed light speed).
    I do not have the physics or the wisdom to understand this, but I have always wondered about it.
    Best regards,
  12. Mar 27, 2011 #11


    User Avatar
    Homework Helper

    It's mostly velocity that is relative. Assuming an inertial frame of reference, which is reasonable, then acceleration is absolute (ignoring near light speed time dialation effects) (also inertial frames by definition are not accelerating).

    Rotation is also absolute since all the parts of an spinning object (except the infinite thin axis of rotation) experience (centripetal) acceleration, which would provide a means to measure the rate of rotation (if the radius is known, and there's no interference from external forces).
  13. Mar 27, 2011 #12


    Staff: Mentor

    Spinning isn't relative.

    What outermost stars? There is no edge of the universe, even in a spinning universe like Goedel's.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook