How Do We Determine Proper Inertial Frames of Reference?

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Discussion Overview

The discussion revolves around the determination of proper inertial frames of reference, exploring concepts related to acceleration, perception of forces, and the implications of Newtonian and modern physics perspectives. Participants examine the differences between various frames, particularly in relation to the Earth and other objects like cars.

Discussion Character

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

Main Points Raised

  • One participant questions how to determine a proper inertial frame, contrasting the Earth with a car, noting the inability to feel the Earth's acceleration despite its motion.
  • Another participant asserts that an inertial frame is one that is not accelerating, pointing out that the Earth is not an inertial frame due to its rotation.
  • A claim is made that Newton's first law applies only in inertial frames, suggesting a limitation on defining proper frames.
  • Concerns are raised about the inability to feel the Earth's rotation, prompting further inquiry into the nature of acceleration and perception.
  • A suggestion is made to use an accelerometer to determine acceleration, with mention of a Foucault Pendulum as a tool to detect rotation.
  • Another participant elaborates on the effects of the Earth's rotation, explaining that it causes a slight reduction in weight, which is difficult to perceive due to its constant nature and the small magnitude of the effect.

Areas of Agreement / Disagreement

Participants express differing views on the nature of inertial frames and the perception of acceleration. There is no consensus on what constitutes a proper inertial frame, and the discussion remains unresolved regarding the implications of these concepts.

Contextual Notes

Participants highlight the limitations of perception in detecting acceleration and the dependence on specific conditions, such as location on Earth and the nature of forces experienced.

daveed
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how do you determine if something is a proper inertial frame of reference? the Earth is but a car isnt? you can feel a car accelerate, even though you are in it, but you can't feel the Earth accelerate as it changes directions in its orbit.. is there a poitn after which you don't feel accelerations much? but this is just Newtonion, what's the modern view on this?
 
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An inertial frame of reference is one that is not accelerating. Just because you can't feel it doesn't mean it's not accelerating. Sitting here on the surface of the Earth we are NOT in an inertial frame since the planet is rotating and we're rotating right along with it (rotation is a form of acceleration!).
 
Newton's first law is true in inertial frames!
 
then you can't say anything is a proper frame.

why wouldn't we be able to feel the Earth's rotation?
 
Use an accelerometer... a cube containing a mass supported by springs. If the mass is centered, then its "reference frame" is not accelerating. To detect rotation, you might need an array of accelerometers.

(A Foucault Pendulum can detect rotation.)
 
daveed said:
then you can't say anything is a proper frame.

why wouldn't we be able to feel the Earth's rotation?

True. In a sense, the general theory of relativity is based on the concept that you can't (locally) distinguish between a force and an acceleration.

The Earth's rotation has the effect of making you a little lighter than you would be if the Earth were not rotating. You don't "feel" the Earth's rotation for two reasons: First, it is with you all the time so you have nothing to compare it to. Second, if you were to go from the north or south pole to the equator, you would become slightly lighter but the effect would probably be too small to measure.
 

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