From: ttt_heg@web.de   
      
   Am Mittwoch000004, 04.02.2026 um 20:24 schrieb Thomas 'PointedEars' Lahn:   
   ...   
   >>>> Acceleration is absolute in Newtonian mechanics.   
   >>>   
   >>> That is not so.  If two reference frames are non-inertial (in the Newtonian   
   >>> sense), then it is possible that one is not accelerating relative to the   
   other.   
   >>   
   >> It was Maciej Woźniak who used the word "absolute".   
   >>   
   >> In this context, "absolute" means "not relative".   
   >   
   > Of course.  We do not have a difference of definition.   
   >   
   >> Speed is relative because the speed of an object depend   
   >> on in which _inertial_ frame of reference it is measured.   
   >>   > In Newtonian mechanics the acceleration dv(t)/dt of an object   
   >> will be the same independent of in which _inertial_ frame of   
   >> reference the speed v(t) is measured.   
   >   
   > One does not have to {use|be in} an inertial frame of reference for one's   
   > measurements.   
      
   Sure, you can also make measurements in non-intertial frames of reference.   
      
   But still in inertial reference frames it is hard to measure 'true   
   velocity' of the own position, because you need to have something,   
   against which you measure velocity.   
      
   And if you are in a truly inertial situation, where you e.g. float   
   inertial through empty space, you have nothing but yourself as reference   
   to measure distances.   
      
   >> Acceleration is not relative, it is absolute.   
   >   
   > Nonsense does not make sense by repetition.   
      
   But this isn't nonsense at all.   
      
   Acceleration is a measure for change of velocity per unit of time.   
      
   That can be measured without external references.   
      
   >>> That is precisely the case (approximately), when a falling observer is   
   >>> observing a nearby falling object and we assume approximatively that the   
   >>> gravitational field is uniform.  Then that object would be observed by thta   
   >>> observer to be at rest even though it would be accelerating for an inertial   
   >>> observer.   
   >>   
   >> You are in free-fall. An apple is right above your head,   
   >> and will stay there.   
   >   
   > That is why it is NOT accelerated in *that* frame of reference, which is   
   > exactly my point.   
      
   This depends actually upon definition of 'reference frame'.   
      
   If you anchor your coordinate system upon the Earth surface and a point   
   right under the free falling position, you are actually changing your   
   velocity in respect to that point, hence are accelerated.   
      
   But you don't feel it, because the effects of acceleration of the free   
   falling observer and local gravity cancel each other out.   
      
   ...   
      
      
   TH   
      
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