From some Portuguese language textbooks, I learned the following definitions:
- linear motion (movimento linear): motion along a line;
- rectilinear motion (movimento retilíneo): motion along a straight line;
- curvilinear motion (movimento curvilíneo): motion along a curved line.
Judging by those definitions, it's clear that linear includes rectilinear and curvilinear, what seems logical when I look at the words, but it seems that, in English language Physics, linear motion is the same as rectilinear motion.
Should that difference be considered a simple linguistic difference? A divergence in how the original concepts evolved and were reinterpreted in each language? Or, contrary to what seemed to me, does linear also include recti- and curvilinear motions in English?
It was said that a "linear motion" would be simply "motion" if defined the way I described. I think that's true if you have already restricted ourselves to the motion of
particles, since the motion of solid bodies and fluids are not fully described as lines.
I should explain that I found that distinction in Portuguese in the context of
uniformly varying motion. As the motion of a car moving on a road or a float in the stream of a river are usually nonrectilinear but might be considered as at approximately constant speed, the concept of
uniform linear motion (constant velocity magnitude) is used instead of that of
uniform rectilinear motion (constant velocity vector).
It seems that Brazilians use the expression
uniform linear motion to refer to
uniform curvilinear motion (the adjective "curvilinear" is rared used here) while English speakers use it to
uniform rectilinear motion (I guess "rectilinear" is rarely used there, since it's a synonym of "linear" in practice).