Understanding Foucault Pendulums

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Discussion Overview

The discussion centers around the conceptual understanding of Foucault pendulums, specifically addressing why the plane of swing does not rotate with the Earth. It explores the mechanics involved, reference frames, and the behavior of the pendulum at different locations, such as the poles.

Discussion Character

  • Exploratory, Conceptual clarification, Technical explanation

Main Points Raised

  • One participant expresses confusion about whether the pendulum rotates with the Earth upon release.
  • Another participant explains that at the poles, the pendulum's motion can be described as oscillating in a thin ellipse, with the axes of the ellipse remaining fixed in a non-rotating reference frame.
  • This explanation includes the idea that the pendulum oscillates in a straight line relative to the Earth, while that line rotates once per day.
  • A later reply acknowledges a newfound understanding of reference frames, indicating that the explanation was helpful.
  • Another participant inquires about the qualifications needed for the suspension of a Foucault pendulum and questions whether it can move in a "figure eight" pattern.

Areas of Agreement / Disagreement

Participants do not appear to reach a consensus on all aspects of the pendulum's behavior, as some questions remain about its motion and suspension requirements.

Contextual Notes

The discussion includes assumptions about reference frames and the specific conditions at the poles, which may not apply universally to all locations or configurations of the pendulum.

Who May Find This Useful

Individuals interested in the mechanics of pendulums, reference frame concepts, and those exploring the physics of rotating systems may find this discussion relevant.

Daboo
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Simple question - I'm having trouble conceptually understanding why the plane that a Foucault pendulum swings in does not rotate with the earth. I understand how the Earth turns under the pendulum, but isn't the pendulum rotating with the Earth when it's released?
 
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As a simple case, consider the pendulum is at the north or south pole.

If you pull the pendulum away from the vertical, hold it steady relative to the earth, and then release it, you are right that in relative to the non-rotating reference frame, initially it has a "sideways" component of velocity equal to omega.x, where omega is the Earth's rotation speed and x is the displacment.

Because of that, it will oscillate by moving round a very thin ellipse, not back and forth in a straight line. Ignoring the finite length of the pendulum, the motion is just two simple harmonic oscillations 90 degrees out of phase, i.e. x = A cos pt and y = B sin pt where A is very much bigger than B.

But the axes of the ellipse won't rotate with time (relative to the non-rotating reference frame) because there are no forces to make them rotate.

So relative to the relative to the earth, the pendulum oscillates in almost a straight line, and that line rotates once per day.

Hope that helps.
 
What qualifications does the Foucault pendulum suspension need to have? Can such a pendulum ever move in a (very thin) "figure eight"? Neat video.
 

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