The first line of Wikipedia states that Inertia opposes change in velocity. My teachers also told that Inertia opposes change in state of rest or motion.
But I would like to know how?
Suppose a body is at rest as seen from an inertial frame and a net external force is applied then according to the above definition, shouldn't every body oppose the acceleration caused from that force because that force will change its state from rest to motion?
If yes, then why don't we count inertia force when calculating the net acceleration of a body ? i.e why isn't:
$$F_{(net\; external)}= inertial\; force+ ma$$