Newton's 2nd Law,
So, if a body is not accelerating, it will experience no change in velocity. It will therefore remain at rest - or continue to move at a constant velocity.
Newton's 1st Law states, roughly, that an object either remains at rest or continues to move at a constant velocity, unless acted upon by a force.
So his 1st law looks to be a "mere" sub-set of his 2nd.
Seems odd but it may have been done this way to confront, head-on, Aristotle's idea that everything that is in motion is thus because there is some cause (ie a force) that is driving that motion. Who knows? Newton was hardly a team-player :)
Other thing worth stressing, I think, is that the 2nd Law has many manifestations. We start at:
But it's really about the net force on an object, so:
And these are vectors , ie direction matters. A particle moving at constant speed but changing direction is accelerating. Eg uniform circular motion. So:
Interim calculus step:
Really important where mass is not constant:
Consolidating all of the above:
The third law is IMHO the truly interesting insight - it leads to universal conservation of momentum.