From: fortunati.luigi@gmail.com   
      
   Tom Roberts gioved=EC 09/11/2017 alle ore 06:06:17 ha scritto:   
   >> The sacrosanct formula F=ma says that if we exert a force F on the mass ,   
   >> the mass accelerates.   
   >   
   > No formula is "sacrosanct", because EVERY equation of physics is valid ONLY   
   > when the conditions for its derivation are met. This formula, like so many   
   > others, is valid ONLY when values are measured relative to a locally inertial   
   > frame. IOW: before applying a formula you must know that each symbol in it   
   > means, and must be sure to apply and conform to those meanings.   
   >   
   > When B turns its engines on and then uses coordinates in which B is at rest,   
   > there is no expectation whatsoever that the above formula applies, because   
   > those coordinates are not a locally inertial frame.   
      
   Ok.   
      
   Suppose now that there is somewhere a third C spaceship on a certain   
   planet where gravity is accelerating in the same way as B, so that B is   
   firm with respect to C while the spaceship A appears to be accelerated.   
      
   How does C assess the accelerations of A and B and their respective   
   forces?   
      
   --   
      
      
   Luigi Fortunati   
      
   [[Mod. note -- I think the author is trying to describe a scenario in   
   which (a) there is a Newtonian gravitational field at C's position,   
   with the vector "little g" equal to B's acceleration vector, and (b)   
   the initial conditions are such that B and C have zero relative velocity.   
      
   Assuming that this is indeed what the author means, then...   
      
   The answer is that so long as C knows her own position (right next to   
   a planet!), she can look up (from pre-calculated tables and surveys)   
   the local gravitational field's vector "little g" at her position,   
   and use this to correct her raw measurements accordingly.  Once she   
   does this, she assesses the accelerations-of and net-forces-acting-on   
   all the spaceships (A, B, and C) correctly.   
      
   I alluded to this scenario in a moderator's note earlier in this thread,   
   but in retrospect I wasn't very clear.  Let me try again:   
      
   This scenario -- trying to make measurements from a measurement platform   
   (here C) which is actually free-falling in a gravitational field -- was   
   a point of controversy when the use of solely-inertial guidance for US   
   nuclear missiles was first seriously proposed in the 1950s.  At that   
   time, some people argued that (despite the successful use of a simpler   
   form of inertial guidance in the German V-2 [A-4] rocket in World War II)   
   inertial guideance couldn't possibly work because the missile would have   
   no way to directly measure the local "little g" at its position.  It   
   took some years for the correct explanation (the one I described above   
   in the paragraph beginning "The answer") to be accepted by all the   
   groups in the US military and its contractors.  Donald MacKenzie's   
   book "Inventing Accuracy: A Historical Sociology of Nuclear Missile   
   Guidance" has a detailed account of this controversy and its resolution.   
   -- jt]]   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)