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A proper explanation of the "anti-gravity wheel"?

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  1. Aug 15, 2015 #1
    Veritasium made a video on how a flight wheel seemed weightless when spinning:

    He then explained in a following video that the reason why it felt weightless is because while he lifted it, he was also swinging the wheel in the x-y plain, which, combined with the wheel's own rotation around the stick generated a torque, so that gravity doesn't apply a net torque that is as strong as it would before, and thus the person feel like the wheel is less heavy.

    I have a similar wheel myself, when I did the same thing, not only did the wheel feel easier to lift, the torque generated by the combined rotations is actually so strong that it is trying to orient the stick upward.

    As far as I know, it is the combined angular velocity of the wheel's rotation around its stick and the stick's rotation around the hand that produces a torque turning the stick up or down with the hand being the pivot, but I really don't understand why and I really want to.
     
    Last edited by a moderator: Aug 16, 2015
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  3. Aug 15, 2015 #2

    Simon Bridge

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    It is not easily understood and takes a while to get used to.
    It basically a manifestation of the law of conservation of angular momentum.
     
  4. Aug 15, 2015 #3
    Where can I learn about it?
     
  5. Aug 15, 2015 #4

    Nidum

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    Look up gyroscopic couple and gyroscopes .
     
    Last edited: Aug 15, 2015
  6. Aug 15, 2015 #5

    phinds

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    There have been several threads about this here on PF. Try a forum search.
     
  7. Aug 15, 2015 #6
    by "this" do you mean the particular video or the phenomenon? Any keywords? If possible, can you give me the link?
     
  8. Aug 16, 2015 #7

    sophiecentaur

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    What is your level of Maths and Physics? The reason I ask is that this (basic Vector stuff but confusing) is a very hard subject and not the place to leap into without a lot of basic knowledge, imo.
    As to the 'reason' for what you experience, we are very bad witnesses to some events and what you experience (or think you experience) in an experiment like this is so unfamiliar that the consequence is you can easily come to the wrong conclusion about what actually happened. It's the equivalent to an optical illusion - but mechanical because it doesn't make sense.
     
  9. Aug 16, 2015 #8
    Either my English is bad or the way you speak is too sophisticated for me, but I don't think I have enough background knowledge for thorough understanding of what's going on. In this case, can you answer this: what is the relationship between the combined velocities and the resultant torque in the other dimension?
     
  10. Aug 17, 2015 #9

    A.T.

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    The torque from gravity doesn't change. Maybe you should watch the explanation video again:



    And check out this:
    http://hyperphysics.phy-astr.gsu.edu/hbase/rotv2.html
     
  11. Aug 17, 2015 #10

    sophiecentaur

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    Sorry about the posy language.
    I was just making the point that, because the experiment produces this precession effect very strongly, it is a rare thing to come across and we (you and anyone) will not interpret the event accurately. The fact that the wheel on the stick rotates upwards over-rules, in our minds, the fact that we still need to support the weight with our hands. We are just aware that the wheel goes up without being consciously lifted so that's interpreted as the wheel getting lighter.
    Remember, it's a question of Angular Momentum and torque and not 'Velocities'. The cross product of the angular momentum vector of the wheel and the torque that's applied to it by moving it 'sideways', produces a resultant torque at right angles to both those vectors. I did a quick Google of the terms used in this thread and there is a range of websites, at different levels.I recommend that you look at a number of the sites and find one to suit you. It is unlikely that you will get anywhere just by Q and A on PF. Q and A is not an efficient way of learning a topic because you need to be asking the right Q, in the first place. Best to do a bit of independent study before asking such a general question as this.
     
  12. Aug 17, 2015 #11
    That was some good points you made, but I am not sure if it is the angular momentum of the wheel's own rotation and the applied torque that generates the new torque, I performed an experiment myself by pushing the stick into revolution around a central point equivalent to the guy in the video, and a reduced net torque due to gravity was consistently observed until the stick come to rest. I imagine that the wheel, during this revolution, must also had its own angular momentum, and since nothing was supplying additional torque during the revolution, it should be the combined angular momentum that causes this, shouldn't it? Anyways, I will do some research myself now, hopefully I will find something useful. (By the way, someone else said something different, not sure how can I know which is right or maybe they are equivalent in some way:
    Torque = polar moment of inertia * angular velocity of the wheel * angular velocity of the stick)
     
  13. Aug 18, 2015 #12

    A.T.

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    Again, the torque due to gravity isn't reduced. The change of angular momentum due to gravity is the same, but the initial and final angular momentum values are different.

    Try to understand the vectors:
    http://hyperphysics.phy-astr.gsu.edu/hbase/rotv2.html
     
  14. Aug 21, 2015 #13
    I know that the torque due to gravity is the same, but the net torque in that particular direction was indeed reduced, isn't it, or am I missing something?

    Anyways, about the link, I can see the first part being an explanation of how torque due to gravity changes the direction of the angular momentum, but I don't know why the wheel would physically turn so that its stick is still pointing in the same direction of the A.M. arrow (L') instead of starting to nutate around L' s arrow. (Maybe this is just my stupidity, but if there is some explanation please do explain)
    Second thing is, when this process is done "the other way around"--having wheel's own rotation and create horizontal motion to produce "vertical" torque--what would be required in the *x-y plain? Angular momentum? Angular velocity? Constant torque?

    *x-y-z reference frame: forward/backward/left/right in x-y plain, up and down in z-direction
     
    Last edited: Aug 21, 2015
  15. Aug 22, 2015 #14

    A.T.

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    No, why should it? The external froces are still the same, and so is the total external torque.


    See here: https://en.wikipedia.org/wiki/Nutation

    In falling, the top overshoots the level at which it would precess steadily and then oscillates about this level. This oscillation is called nutation.

    In our idealized case of the gyro the precession doesn't change the external torque magnitude, so it cannot overshoot. It can precess at a constant rate due to a constant external torque, relative to the angular momentum vector.
     
    Last edited: Aug 22, 2015
  16. Aug 22, 2015 #15
    Do you think the combined rotations would generate torque in the "vertical" direction?
     
  17. Aug 22, 2015 #16

    jbriggs444

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    That sentence is not sensible. Rotations do not generate torque. Forces produce torque. The convention for direction with torques is that a "vertical" torque generates rotation in the horizontal plane. That is, the direction of a torque is taken to correspond with the direction of the axis of rotation.

    The combination of a rotation about a horizontal axis with a precession about a vertical axis requires the existence of a torque about the horizontal axis at right angles to both. In the case at hand, that torque is provided by a "couple" -- a pair of equal and opposite forces acting at different points, gravity pulling down on the wheel and the person holding up the end of the shaft.
     
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