Conservation of momentum in gyroscopes

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Discussion Overview

The discussion centers around the conservation of momentum in gyroscopes, particularly in the context of a single-axis gyro and its behavior when a force is applied. Participants explore concepts related to pitch change, precession, and the effects of external forces, including aerodynamic forces, on the motion of gyroscopes and similar objects like thrown footballs.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant questions whether a gyro will continue to change pitch after a force is applied and then removed, asking about the conservation of momentum in this scenario.
  • Another participant explains that applying a force results in linear motion in addition to changing pitch, indicating that the pitch change is a precession without associated angular momentum, suggesting there is no momentum to conserve.
  • A later reply seeks clarification on whether the pitch remains constant after the force is removed, to which another participant confirms that it does.
  • One participant introduces an analogy with a thrown football, asking how the pitch continues to change after the force of the throw has subsided.
  • Another participant speculates that the gyroscopic effect may not dominate due to insufficient spin speed, while also expressing uncertainty about the presence of nutation in rotating objects.
  • It is noted that aerodynamic forces continue to act on the thrown football, contributing to the change in pitch.

Areas of Agreement / Disagreement

Participants express differing views on the conservation of momentum in the context of gyroscopic motion, with some agreeing on the lack of angular momentum associated with pitch change, while others raise questions about the effects of external forces and the behavior of rotating objects. The discussion remains unresolved regarding the implications of these forces and the nature of motion in gyroscopes and similar objects.

Contextual Notes

Participants acknowledge the complexity of the topic, with some noting that certain concepts may be beyond the scope of introductory physics education. There are also mentions of unresolved aspects regarding the behavior of rotating objects that are not spherically symmetric.

gyroguy
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Not a scientist, please be nice :)

Let's assume I have a singe axis gyro (flywheel) spinning in space. I apply a force to a point which results in a change in pitch. I apply this force until the gyro is at 5* pitch and then stop.

Will the gyro continue to change pitch after the force stops?

If not, is the momentum from the pitch change conserved? How so? Or is there no momentum to conserve?

Thanks!
 
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If you apply a force (as opposed to a "couple") then the gyroscope will move linearly in the direction of the force in addition to changing pitch. It will continue to move linearly after the force is removed.

The changing pitch resulting from the applied force is a precession rather than a rotation. It has no associated angular momentum. So yes, there is no momentum to conserve.
 
jbriggs444 said:
If you apply a force (as opposed to a "couple") then the gyroscope will move linearly in the direction of the force in addition to changing pitch. It will continue to move linearly after the force is removed.

The changing pitch resulting from the applied force is a precession rather than a rotation. It has no associated angular momentum. So yes, there is no momentum to conserve.

So I am understanding that the pitch would remain the same 5* and the gyroscope would continue to move linearly?
 
gyroguy said:
So I am understanding that the pitch would remain the same 5* and the gyroscope would continue to move linearly?
Yes.
 
Can you expand on this for something like a thrown football? When a football is thrown with the proper spiral the pitch of the nose will follow the arc. How does the pitch continue to change after the force of the throw has subsided?
 
Humm. Good question. I suspect that they don't spin fast enough for the gyroscopic stabilising effect to dominate other forces such as aerodynamic forces.
 
CWatters said:
Humm. Good question. I suspect that they don't spin fast enough for the gyroscopic stabilising effect to dominate other forces such as aerodynamic forces.
That is my suspicion as well. However, I am unable to convince myself that there is no nutation going on. An object that is rotating and which is not spherically symmetric need not move in a pure rotation around its axes of symmetry (if any). It can spin and wobble. The angular momentum of such an object remains constant, but its orientation can have the visual appearance of undergoing a slow rotation.

[Mind you, this is outside the scope of a first year physics education which is the only formal training I've had]
 
gyroguy said:
How does the pitch continue to change after the force of the throw has subsided?
There are still aerodynamic forces acting.
 

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