But 0 is what you expect to see in that case: you have two forces acting on the body, of equal magnitude and in opposite directions, so the net force is 0, so a

But 0 is what you expect to see in that case: you have two forces acting on the body, of equal magnitude and in opposite directions, so the net force is 0, so acceleration is 0. But if you jump out of a plane, you will see ~0. If gravity were a force, what would be the equal and opposite force acting on you?