Therefore, long-range forces cannot act on an object in a vacuum. An impetus can be imparted by an applied force and transmitted from one object to another. An impetus may wear out or build up in the same way as the effect of an applied force. Resistance opposes an applied force or consumes the impetus of a moving object. Obstacles may redirect or stop motion, but they cannot be agents of an applied force.
Gravity is a tendency of objects to fall down.
In this conception, gravity is not necessarily a force. Nevertheless, the causal principles for applied forces which we quoted above may be attributed to gravity as well. As we saw in our discussion of Aristotelian physics, an important consequence of those principles is the belief that heavier objects fall faster. This belief is so common that it deserves to be examined carefully in physics classes. So the best teaching strategy may be the direct one that aims at convincing students that gravity is a force, in particular a long-range force.