I throw a metal rod (not light) into the air

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    Air Light Metal Rod
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Discussion Overview

The discussion revolves around the dynamics of a solid cylindrical metal rod thrown into the air, focusing on its degrees of freedom in motion, including rotational and translational aspects. Participants explore the implications of angular velocity and the effects of rotation on the rod's trajectory, while also considering the complexities introduced by additional degrees of freedom.

Discussion Character

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

Main Points Raised

  • One participant describes the rod as having uniform density and spinning with angular velocity omega, questioning the total degrees of freedom beyond this.
  • Another participant identifies three degrees of freedom: roll, pitch, and yaw.
  • A participant raises the question of whether the rod's projectile motion should be treated separately from its rotational motion.
  • It is suggested that the center of mass of the rod follows a parabolic trajectory, although the impact of high-speed rotation on this motion is uncertain.
  • Discussion includes a mention of rotational inertia and its potential effects, particularly in relation to off-center forces and precession.
  • One participant recalls an experiment involving a spinning bicycle tire to illustrate the effects of rotational dynamics.
  • Another participant notes that for an object in freefall, the rotational degrees of freedom may reduce to a single effective angular velocity about one axis.

Areas of Agreement / Disagreement

Participants express differing views on how to account for the rod's motion, with some suggesting that rotational dynamics complicate the analysis, while others propose simpler models. The discussion remains unresolved regarding the exact treatment of the rod's motion and the implications of its rotational dynamics.

Contextual Notes

Participants acknowledge limitations in their understanding of the effects of rotation on projectile motion and the complexities of calculating moments of inertia with multiple degrees of freedom. There are also references to specific physical effects that may not be fully clarified.

jewbinson
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It is cylindrical in shape and is solid (not hollow) and has uniform density. Igonre air resistance.

It spins with some angular velocity omega and spins in the other possible ways.

Apart from omega, how many more degrees of freedom is possible?

I can find the MOI of a rod spinning with just angular velocity omega on a horizontal table, and hence the Lagrangian (and Hamiltonian).

But what about with these extra DOF?

I'm not familiar with working out the MOI for more than 1 DOF.

Thanks for your help in advance.
 
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there are 3 total --- roll, pitch, and yaw
 
Ok... do we treat the motion of the rod as a whole (projectile motion) separately or is that included in our calculation? Can someone please point me in the right direction of where to start? I have no clue...
 
The center of mass of the rod doesn't care about the rest of the motion, it travels in a parabola. This is a bit simplistic, probably, since if the cylinder is rotating at a high speed I'm not sure what the effect of that is, but there may be one ... I don't remember the physics of it.
 
Sounds like it might be a magnetic effect you're thinking of... if not then I would be interested as to what the effect you are thinking of is.
 
jewbinson said:
Sounds like it might be a magnetic effect you're thinking of... if not then I would be interested as to what the effect you are thinking of is.

No, not magnetic, it's an effect due to rotational inertia. I think it may not appy here since I seem to recall that it only has an effect when a roating cylinder is subjected to a an off-center force. The classical physics class experiment that demonstrates what I'm talking about is where you hold the free-spinning axle of a bicycle tire and have someone rev it up quite a bit and then you try to move one side of the axle towards you and the other away from you. It doesn't go in the direction you expect, it moves differently. Might be called precession. It's been about 50 years since I saw the experiment and I don't do physics.

EDIT: Might be the effect happens not from an off-center force so much as any force that tries to change the direction of the axis of rotation.
 
I think I might have come across what you describe.

If you spin an empty bottle of water in suspended air ("roll"), the angular velocity decreases with time
 
jewbinson said:
It spins with some angular velocity omega and spins in the other possible ways.

Apart from omega, how many more degrees of freedom is possible?

phinds said:
there are 3 total --- roll, pitch, and yaw

For an object in freefall, I would think that the number of degrees of rotational freedom is 1. I.e., it doesn't matter what the object's angular velocity is about the x, y and z axes—these perpendicular angular velocities always resolve into a single angular velocity about a single axis of rotation. (I could be wrong, however...)
 

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