Comment by omnicognate

12 hours ago

1. Unlike position and velocity, which are relative (there is no given "origin" for them, no way to say where a thing is or how fast it's moving except relative to other things), rotation is absolute. A thing is either rotating or not, regardless of its relation to other things. Objects that rotate "experience (centrifugal) forces as a result" or "require (centripetal) forces to hold them together" depending on how you choose to describe it. This is detectable: hook two weights together with a newton-meter in space and the newton-meter will read non-zero when the assemblage is rotating, zero when not. The reading tells you how fast it is rotating regardless of any external reference point. (An equivalent device to detect position or velocity is not possible, but it is for acceleration.)

2. Yes, everything "at rest" on earth is in fact rotating at the rate the earth rotates. If you stand on the equator at midday and do not rotate you will be standing on your head at midnight.

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omnicognate

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charcircuit  12 hours ago

>no way to say where a thing is except relative to other things

This is always true. The origin is just a thing that other things are relative to. It's just as possible to define an origin in the real world as it is on a piece of graph paper.

unhba  11 hours ago

Thanks for this explanation. If I understand correctly then, the moon requires some centripetal force in order not to dissipate due to its rotation whereas e.g. my head or the Eiffel Tower do not because they are not subject to absolute rotation.

  • card_zero  11 hours ago

    They're rotating too.

    If you rotate as part of some larger rotating thing then you still rotate. (You also move around.) It's all absolute.

    • omnicognate  11 hours ago

      Indeed. The Eiffel tower and your head do both have some (extremely small) centripetal force compensating for their rotation along with the earth.

      (You can break that down in different ways, i.e. use various choices of generalised coordinates to describe it, so exactly what constitutes "centripetal", "centrifugal", "gravitational", "tidal", etc. forces depends on that. I'm being pretty vague in how I decribe it. Regardless, rotation is absolute, or in other words the equations of physics take a different form in a rotating frame of reference than in a non-rotating one.)

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