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Re: non-inertial: part II



What is a force? As I understand Newton, and I haven't read the original,
is that which causes an object to deviate from motion in a straight line
at a constant speed (or a change in momentum) in an inertial frame,
presupposing the existence of, and knowledge of, such frames of
reference. Forces are measured by m*a in any inertial frame, not by
presure gages - the physics consists of identifying those situations,
interactions, in which accelerations are found, and how the force, m*a,
is related to parameters of the situation. Spring balances, presure
sensors, strain gages etc. are convenient tools for measuring force, not
its definition.
If this understanding is correct, force is tied to inertial frames. In
non-inertial frames, we can introduce apparent forces to account for the
acceleration inherent in the frame. General Relativity then transforms
gravity into one of those apparent forces.
Also, in this view, force is fundamentally different from acceleration,
because it is the change in the physical quantity momentum, not just the
kinematical quantity acceleration.

Al Clark