The action of a resistive medium or an obstacle is not an active force, because it does not start or sustain motion. It may, however, be called a reactive force , to help students develop a general force concept. Newton's third law is inconsistent with common sense intuitions. Motion is determined by the larger of two competing forces. A student needs to reinterpret this experience to accept Newtonian theory.
Motion is determined by a compromise among competing forces.
Of course, the superposition principle can be regarded as a kind of compromise, but student ideas of compromise are likely to be vague or involve an impetus. Sometimes dominance and compromise principles are used together, as we saw in connection with Fig. 4. An applied force is a push or pull exerted by an agent in direct contact with the object. For some, only the living things are recognized as agents of force. Weight is not distinguished from mass.