's first law
in view of the second law
: since indeed F=ma
, if no force
is acting (and the object
), 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
of a natural phenomenom
s aren't observable physical object
s, 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.