If an object is at rest on a flat surface I remember my school teacher would say, "By Newton's 1st law the normal reaction, R, is mg (mass x g)". Ok, sure this is fine because the object is at constant velocity (velocity=zero) and so there are no net forces and so the two forces must balance. But what's wrong with saying "By Newton's 2nd law R-mg=0. Therefore R=mg." (The net force on the object is zero, therefore the acceleration on the object is zero) But can't you also say, "By Newton's 3rd law R=mg." (The object is pushing on the ground with a force of mg, therefore the ground is pushing on the object with a normal reaction of mg).
Is this correct or have I erred somewhere?