http://www.ck12.org(Taken from the Principia in modern English, Isaac Newton, University of California Press, 1934).
Understanding motion comes in two stages. The first stage you’ve already seen: you can figure out where something
will go, and how fast it will get there, if you know its acceleration. The second stage is much more interesting:
where did the acceleration come from? How can you predict the amount of acceleration? Mastering both stages is
the key to understanding motion.