As you noted, all those equations are only true when the acceleration "
a" is a constant. If all the motion is along some particular line, you can treat the variables
v (velocity),
a, and
s (displacement) as simple numbers describing motion along that line. You do have to be careful to describe one direction as positive and the other as negative and stick to that rule for each variable. The equations still work when
v,
a, and
s are vectors, so long as "
a•s" is taken to mean the vector dot product. When
a and
v aren't along the same line, then the direction of motion changes and the object follows a curving path. Maybe some textbooks don't use vector notation because they are trying to introduce the simpler case first.
Many interesting types of motion (e.g. circular orbits) are not described by these equations because
a isn't constant.
Mike W.
(published on 12/22/2010)
