Motion Along One Dimension

The definitions of average and instantaneous velocity, speed and momentum set up in §2.9 hold for motion in any number of dimensions, in full vector form. Restricting attention to motion confined to a single straight line — one-dimensional motion — simplifies all of these: with only one axis in play, a velocity or acceleration is completely captured by a single signed number (a $+$ or $-$ sign standing in for 'which of th …