Newton's first
law of
motion seems
redundant in view of the
second law: since indeed
F=ma, if no
force is acting (and the
object has
mass), then its
acceleration must be
0, and it must keep its current
velocity.
So why is it there?
Newton's laws of motion
aren't a
description of a
natural phenomenom.
Forces aren't observable
physical objects, except by application of Newton's laws. Instead, the
3 laws of motion can be considered an
axiomatic definition of what force
is.
The first law just tells you how to observe a force: just look for any object with a non-constant velocity. Now that you know what a force is, you can proceed to measure it using the second law.
Newton was well-versed in the axiomatic method; his 3 laws are a nice example of how it can be used.
Either that, or Newton thought he'd look silly with second and third laws, but no first law to back them up.