Question regarging pendulum axis and its motion

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    Axis Motion Pendulum
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Discussion Overview

The discussion revolves around the motion of a pendulum with a horizontal bar pivoting around an axis, specifically examining how the distance of the pivot point affects the pendulum's motion and period. Participants explore the implications of neglecting the weight of the bar and consider the differences between two configurations of the pendulum.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant questions how the distance of the pivot axis (x and y) influences the motion of the pendulum, particularly whether one configuration will swing for a longer period than the other when released from the same position.
  • Another participant suggests that the equations of motion for the pendulum may not include the lengths x and y, indicating that the pendulum primarily moves in the z plane.
  • A participant expresses uncertainty about applying standard pendulum models to their specific setup, which differs from traditional pendulum designs.
  • It is proposed that if the longer bar bends more, it could dissipate energy more quickly, potentially affecting the period and motion of the pendulum.
  • One participant shares a gut feeling that neglecting the weight of the bar may not significantly affect the motion, but raises curiosity about the impact of the bar's weight in their specific case.

Areas of Agreement / Disagreement

Participants express differing views on the influence of the pivot distance and the weight of the bar on the pendulum's motion. There is no consensus on whether these factors will significantly alter the period or behavior of the pendulum.

Contextual Notes

Participants acknowledge limitations in their descriptions and the complexity of accurately modeling the pendulum's behavior, especially regarding higher-order effects such as bending and friction.

callekula
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Hi,

I have a question regarding a pendulum and it's motion/momentum given the axis it moves around.

In the below picture, there are two version of a pendulum with a weight at the bottom. It moves in and out of the screen, around an axis resting on two nails (red). My question is, how does the distance (x and y) of the axis affect the motion, if we neglect the weight of the horisontal bar? Will the x-version move for a longer period of time, if the pendulum is released from the same position as the other version? Or is there no difference?
pendulum.png
 
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Hello callekula, :welcome: !

In a physics forum you can expect a physics answer; in PF, however, we really value the input from the poster: what do you think ? And instead of a direct answer (a spoiler, so to say) you get guiding questions an little nudges to help you find the answers you are looking for. :wink:

To help you on your way: if you had to write down the equations of motion for these pendula, would your lengths appear in those ?
 
Hi BvU,

I appreaciate your willingness for me to figure it out myself. However, the pendulum I am building does not look like the ones in the school books with formulaes attached to them, there they are often depicted hanging in a string in one fixed point. This pendulum swings using this horisontal bar that rests on two screws, so I could never figure this out myself I think.
 
Seems to me you are able to do the experiment !
My reply was 'first order': pendulum moves in the z plane and your x and y don't appear in the equations of motion.
For an accurate clock mechanism you'd need a much more detailed description to estimate higher order effects (bending, friction, ...)
 
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Sure, I'm not expecting to get an exact answer with such a simple problem description. My gut feeling, if the weight of the bar is neglected, is that it won't make any difference. But in my actual case it does have a weight, although very small (a wooden bar, 2 cm in diameter). So I'd be curious if it had a notable effect of the motion over a period of time, if this particular bar would be a factor say 6 times longer. I'd definitely do experiments, but was just reaching out if someone already knew :-)
 
callekula said:
Will the x-version move for a longer period of time, if the pendulum is released from the same position as the other version?
View attachment 248152

If the longer bar bends more, it will dissipate energy quicker. The period might also be different due to that and the greater moment of inertia.
 
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