Will a free falling rod rotate or not?

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    Falling Rod Rotate
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Discussion Overview

The discussion centers on whether a free falling rod will rotate or not, particularly in relation to the choice of the rotational center. Participants explore the implications of selecting different points as the center of rotation and the resulting moments acting on the rod. The conversation touches on concepts of rotational dynamics, equilibrium, and the parallel axis theorem.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant questions why the center of gravity is typically chosen as the rotational center, suggesting that if another point is chosen, a net moment due to gravity would cause the rod to rotate.
  • Another participant asserts that if the center of gravity is chosen, the rod will not rotate due to the absence of a net moment acting on it.
  • Some participants mention that using a point other than the center of gravity complicates the analysis, requiring the application of the parallel axis theorem.
  • A participant argues that regardless of the chosen point for rotation, the result should remain consistent, implying that the rod will not rotate.
  • There is a question about how to consider net moments acting on a bicycle when selecting points related to friction and normal reactions.

Areas of Agreement / Disagreement

Participants express differing views on whether the rod will rotate based on the choice of rotational center. Some argue that it will not rotate if the center of gravity is used, while others contend that selecting different points could lead to rotation. The discussion remains unresolved with multiple competing perspectives.

Contextual Notes

Participants reference concepts such as net moments, torque, and fictitious forces in the context of accelerating bodies, but the discussion does not reach a consensus on the implications of these concepts for the rotation of the rod.

peterpang1994
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Will a free falling rod rotate ?

When we consider a bicycle is turning on a flat plane, we know that there is friction, which provide centripetal force on the bicycle. And we know that the bicycle is no longer perpendicular to the flat plane so as to reach equilibrium. What I want to ask why we always choose the center of gravity of the bicycle be the rotational center. Just like when a rod is free falling and parallel to the ground, if I choose the center of gravity as the rotational center the rod will not rotate, because there is no net moment acting on the rod. But if I choose the points other than the center of gravity as the rotational center, there would be net moment due to gravity and the rod will rotate. Will the rod rotate or not? Why we always choose center of gravity as the rotational center?
 
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peterpang1994 said:
And we know that the bicycle is no longer perpendicular to the flat plane so as to reach equilibrium.

So no one rides a bike upright?
 


peterpang1994 said:
But if I choose the points other than the center of gravity as the rotational center, there would be net moment due to gravity and the rod will rotate. Will the rod rotate or not? Why we always choose center of gravity as the rotational center?

We choose the center of gravity for simplicity. If you use another point in the body for the center of rotation, you have to use the parallel axis theorem.
 
But when I choose the other point as rotational centre and chage the moment of inertia by parallel axis therom, there are probabilities for the rod to rotate or not.
 


peterpang1994 said:
if I choose the center of gravity as the rotational center the rod will not rotate, because there is no net moment acting on the rod.
Ok
But if I choose the points other than the center of gravity as the rotational center, there would be net moment due to gravity and the rod will rotate.
There would NOT be net moment.
Imagine a massless rod AB. On the extreme B of the rod let's put a weight.
Let's put the rod horizontal and let it fall.
As said before, if I choose B as center of rotation, the rod will not rotate.
If I choose A, the forces acting of B will be mg pointing downwards, and ma pointing upwards. Of course a=g
That's because the body is accelerating, and with accelerating bodies you should always consider the fictious force ma.
Again, no net torque.
Will the rod rotate or not?
Of course not. The result must be the same regardless the method you use to study it.
Why we always choose center of gravity as the rotational center?

As you see, you can choose any point you like.
 
Thank you very much .In the case of bicycles, how should I consider the net moment acting on the bicycle if I choose the point where the friction and the normal reaction acting on.
 

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