When someone leaves an outside door open on a cold, still day, it is often said that the person is letting in cold air. If so, how

Diffusion

Explanation:

This process can be referred to as diffusion. The cold air moves from a region of high concentration (outside) to a region of low concentration (inside). The movement of these cold air is what is referred to as "letting in cold air".

NOTE: Diffusion is the process in which substances move from a region of high concentration to a region of low concentration. Just like when an insecticide is sprayed inside a closed room and then the room is later opened so as to allow the the sprayed insecticide leave the room.

air exerts forces on falling objects near earth’s surface.

explanation:

the term 'free falls' refer to object that are falling towards the earth, in a situation in which gravity is the only force acting on the object.

for an object in free fall, the acceleration is constant (g, the gravitational acceleration), and so the velocity of the object constantly increases.

for objects near earth's surface, there is always another force acting on the object: the resistance due to the air. this force is opposed to the direction of fall of the object, and it is proportional to the velocity of the object, so as the object falls down, this force increases up to a point when it balances the force of gravity, and the object continues its motion at constant velocity (called terminal velocity). since in this case gravity is not the only force acting on the object, we are not in a situation of free fall.