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Can a spherical spinning projectile oscillate in flight?

  1. Nov 7, 2012 #1
    There is a very heated debate in a forum for airsoft that I frequent. It is about what happens to ba bb pellet when it is fired from a bb gun. Two point of contention have formed.

    1. A unbalanced bb, who's geometric center is disparate from it's center of mass will wobble, or oscillate on the y and x axis due to it's imbalance.

    2. The opposing view is that all spinning objects spin around it's center of mass and travel a linear parabolic path and thus it is impossible to wobble.

    So which is true?

    Here is a link to the thread of that discussion.


    Thanks for your responses!
  2. jcsd
  3. Nov 7, 2012 #2


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    If the BB is unbalanced, it should spin about it's center of mass. However due to turbulence, which could be further aggravated by a unbalanced spinnnig bb, there will be some "wobble". If there is a component of deflection of the air perpendicular to the path of the bb, then the center of mass of the bb will be accelerated a small amount in the opposite direction (also perpendicular to the path of the bb).
  4. Nov 8, 2012 #3


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    What happens in the gun barrel? Are they rifled to make the BB spin? If so then presumably it will try to spin about the center of mass but it will be constrained by the barrel. My guess is the mass of the gun forces it to spin about the center of the barrel rather than the center of mass but that might not be true. Then when it leaves it will spin about it's center of mass as others have said. I wonder what happens in the transition? Either way it sounds like balanced BB are better.
  5. Nov 8, 2012 #4


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    What exactly will oscillate? The geometric or the mass center?

    What exactly does "wobble" mean? If the objects geometric and mass center are different, then even if the mass center moves parabolically, the geometric center can oscillate, causing a visual wobble effect.

    The assumption that the mass center must move parabolically is true for vacuum. Aerodynamics makes it more complicated, as rcgldr pointed out. An off-mass-center aerodynamic force will cause a torque that tries to change the axis of rotation. The gyroscopic effect causes precession of the rotation axis, allowing a cyclic acceleration of the mass center perpendicular to the flight path by the Magnus effect. And turbulence makes it even more complicated, because under certain conditions it can resonate and create aerodynamic flutter.
  6. Nov 8, 2012 #5
    What is tracked in a spinning projectile? It's center of mass or it's geometric center?
    In a hypothetical vacum, given two objects with the same mass and velocity, will imbalanced object distance traveled be less than the balanced one? If the geo center was tracked it would be obvious that the unbalanced object will travel less because of the wobble but it wouldn't matter if center mass was tracked.

    The argument was the would be movement decay in the horizontal axis caused by vertical oscillations of the geo center not necessarily caused by turbulence.

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