The world revolves around a person

  • Context: Undergrad 
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Discussion Overview

The discussion explores the concept of relative motion, particularly whether one can say that the world revolves around a person or object. Participants consider perspectives from relativity and the implications of non-inertial frames, such as the Earth's rotation.

Discussion Character

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

Main Points Raised

  • One participant suggests that while people typically think of objects moving relative to the Earth, it may also be valid to consider the Earth moving relative to a stationary object, like a car.
  • Another participant confirms that the Earth is not an inertial frame due to its rotation, which complicates the discussion of relative motion.
  • A question is raised about whether relative motion can only be discussed when objects are not accelerating or when they do not have angular rotation about a common axis.
  • It is noted that an inertial observer should not experience linear acceleration or rotation, but most laboratories are non-inertial due to the Earth's rotation and gravity, which introduces fictitious forces.
  • Participants mention that while fictitious forces can complicate the analysis, they are often small enough to be ignored in practical scenarios.

Areas of Agreement / Disagreement

Participants generally agree on the non-inertial nature of the Earth and the implications for discussing relative motion, but there are differing views on the conditions under which relative motion can be accurately described.

Contextual Notes

The discussion involves assumptions about inertial frames and the effects of gravity and rotation, which may not be fully resolved. The complexities of defining motion in non-inertial frames are acknowledged but not conclusively addressed.

silentwf
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This isn't the phrase egotistic or anything, but rather a random conversation i came up with my friends. We all know that people (and things) revolve around the world, but could one say that the world revolves around a person/thing? For example, if a car crashed into a wall, one could picture it as the car moving towards the wall, but from another perspective, one could view it as the wall moving toward a stationary car.

I know that in relativity, things must be in the same inertial frame, but the Earth is rotating, which isn't an intertial frame... so is it wrong to say the world revolves a person/thing?

(i'm not sure if this goes in general physics or relativity)
 
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hi silentwf! :wink:
silentwf said:
I know that in relativity, things must be in the same inertial frame, but the Earth is rotating, which isn't an intertial frame... so is it wrong to say the world revolves a person/thing?

yes, you're correct … the Earth isn't an inertial frame :smile:

(we usually treat it as such, but that's not always accurate)

let's keep it simple by assuming that you are standing at the north pole …

if you can somehow position yourself on a platform which is not rotating with the earth, you would look down and see the Earth rotating about you

moreover, that would not be a merely permissible view, it would be the natural view to take, since you would have the inertial frame (i'm ignoring the motion round the sun, and round the galaxy etc), not the earth

at ordinary latitudes, it's not so straightforward, since your platform would have to roate around local vertical, but basically the reality is that the Earth is turning, and it doesn't make much difference which (parallel) axis we regard it as turning about
 
So basically, you can only say things move in relative to another only when they're not accelerating? or is it only when they don't have any angular rotation about a common axis?
 
an inertial observer (or an inertial frame) should have neither linear acceleration nor rotation

however, most laboratories are rotating with the earth, and going round the sun etc, and even if we adjust for that, the Earth's gravity would still make the laboratory non-inertial, requiring us to add a "fictitious" force of gravity (which we're so used to that we regard it as real! :wink:)

ultimately, you can have any non-inertial frame, so long as you introduce "fictitious" forces …

but often those forces are so small that they can be ignored! :smile:​
 

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