From: ttt_heg@web.de   
      
   Am Donnerstag000029, 29.01.2026 um 21:31 schrieb Paul B. Andersen:   
   > Den 28.01.2026 21:02, skrev Maciej Woźniak:   
   >> On 1/28/2026 8:04 PM, Paul B. Andersen wrote:   
   >>> Den 28.01.2026 07:56, skrev Maciej Woźniak:   
   >   
   >>>>   
   >>>> BTW. You wrote:   
   >>>>   
   >>>> 1)an object is always at rest (velocity 0)   
   >>>> in its rest frame   
   >>>> 2)proper acceleration of an object is its   
   >>>> acceleration  measured  in its rest frame   
   >>>>   
   >>>> 1+2=3) the proper acceleration of any   
   >>>> object is always 0.   
   >>>> Sorry, poor trash.   
   >>> Now I know that you still think that if a spacecraft   
   >>> with mass 1000 kg is pushed by a rocket engine with   
   >>> the force F = 10000 N, then the proper acceleration of   
   >>> the spacecraft is a = 0  because the speed of the spacecraft   
   >>> relative to itself is 0 m/s.   
   >   
   >>   
   >> Sure, and a proper shark eats grass.   
   >>   
   >> Still, 1+2=3. According to the definition   
   >> you've invented  the proper acceleration of   
   >> any object is always 0. Sorry, poor trash.   
   >   
   > :-D   
   >   
   > Given v(t) = kt where k is a constant, -∞ < t < ∞   
   >   
   > What is v at t = 0?   
      
   What is 't=0' ?   
      
   Time is actually an interval and requires a beginning and an end.   
      
   In common dates we treat the birth of Jesus as beginning of our time   
   intervals.   
      
   But in most cases we use much closer events and start our clocks at some   
   points, where we beginn a certain experiment or measurement.   
      
      
   Anyhow..   
      
   The velocity 'v' is depending upon the reference point. If that   
   reference point is actually moving, we could still use it as reference   
   and define 'velocity' as 'change of relation to that point'.   
      
   Usually we also need 'direction' and therefor need a coordinate system,   
   which is directed in an appropriate direction and has the reference   
   point in the center.   
      
   Now we get a velocity in respect to that coordinate system, if we   
   consider the velocity of something in respect to that system.   
      
   Which velocity we ascribe to that object is mainly a question of taste   
   and convenience, because we could chose any coordinate system we like.   
      
   But from 'constant velocity' we cannot derive velocity, but for   
   'inertial motion' we could chose any velocity we like.   
      
   However, the best choice would be v=0 for the coordinate system itself.   
      
   (even if that is not necessarily the case)   
      
   ...   
      
      
   TH   
      
   --- SoupGate-Win32 v1.05   
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