From: goldenfieldquaternions@gmail.com   
      
   On Monday, April 29, 2019 at 5:03:41 PM UTC-5, Luigi Fortunati wrote:   
   > The second principle F=ma states that if we apply the net force F   
   > to  the mass  of the wheeled cart, the cart accelerates.   
   >   
   > And so far we all agree, I think.   
   >   
   > Is the opposite also valid? In other words, is it correct to say   
   > that  if NONE applies force to him, the trolley does NOT accelerate?   
   >   
   >   
   > [[Mod. note --   
   > Yes, provided that we're (making all the measurements) in an   
   > inertial reference frame.  See   
   >   https://en.wikipedia.org/wiki/Inertial_reference_frame   
   >   https://en.wikipedia.org/wiki/Newton%27s_laws_of_motion   
   > for more details.   
   > -- jt]]   
      
   This gets a bit strange with curved spacetime and general relativity. In   
   this case you can be two masses in inertial frames. but they are not the   
   same Lorentz frames. As a result while there is no force acting on the   
   two particles the vector separating them may change in a nonconstant   
   manner.   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)