From: PointedEars@web.de   
      
   Paul B. Andersen wrote:   
   > Den 04.02.2026 01:06, skrev Thomas 'PointedEars' Lahn:   
   >> Paul B. Andersen wrote:   
   >>> Den 03.02.2026 16:14, skrev Thomas 'PointedEars' Lahn:   
   >>>> Paul B. Andersen wrote:   
   >>>>> Den 03.02.2026 10:07, skrev Maciej Woźniak:   
   >>>>>> Anyway, for any context motion is absolute.   
   >>>>> Speed is relative, acceleration is absolute.   
   >>>> Wrong.   
   >>>   
   >>> 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.   
      
   > Acceleration is not relative, it is absolute.   
      
   Nonsense does not make sense by repetition.   
      
   >> 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.   
      
   > (If you are falling towards the Earth, the distance between your   
   > head and the apple will increase very slowly. We ignore it.)   
   >   
   > You will know that since you are in free-fall your and the apple's   
   > proper acceleration is exactly zero. (no air resistance)   
      
   According to the Einstein equivalence principle it is not possible to   
   distinguish, without external reference, free fall in a uniform   
   gravitational field from a state of rest not under the influence of   
   gravitation, or motion in an accelerated reference frame not under   
   the influence of gravitation from motion in a frame at rest under   
   the influence of uniform gravitation.   
      
   So it is not correct to say "acceleration is absolute".   
      
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   PointedEars   
      
   Twitter: @PointedEars2   
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