Air Resistance

Friction isn’t the only force that tends to slow moving objects down. The air itself has a similar effect on objects, exerting a force called air resistance or drag. Everything moving through an atmosphere experiences air resistance to one degree or another; you’ve felt it if you’ve ever walked against a strong wind. Some objects experience air resistance more than others, depending on its density and shape: for example, a flat piece of paper will drift slowly to the ground while a crumpled piece of paper experiences less air resistance and falls much faster. Parachutes exploit air resistance to fall more slowly than a rock.

It is customary in introductory physics to ignore air resistance when solving problems, and we will continue that custom here. That is not to say that air resistance is insignificant; to the contrary, it can be very significant as some of the examples above demonstrate. We ignore it, rather, because air resistance is hard to work with, mathematically. Not only does it depend on the shape of the object experiencing it, but it also depends on the object’s velocity: the faster an object moves through the air, the larger the force it feels. For technical reasons this makes the mathematics a lot more difficult to do. So we ignore it, remembering that things travelling through air will be a little slower, and travel a shorter distance, than our equations predict.