Air resistance is caused by friction between a moving object and the air molecules surrounding it. The energy required to push these molecules out of the way diminishes the object's kinetic energy and correspondingly reduces its velocity.
Aerodynamics is the study of the forces that affect objects moving through the atmosphere. The air resistance experienced by a moving object is known as drag. As an object's velocity increases, its drag also increases through the displacement of an ever-greater volume of air. A continuously accelerating object eventually reaches the point at which its drag and weight are equal. Such an object has reached its terminal velocity, and further acceleration is impossible.
An object's shape significantly affects the amount of drag it generates. Objects with large surface areas relative to their weight, such as feathers and snowflakes, have very high drag coefficients, the quantity of drag as expressed in a mathematical formula. It follows that an object with a high drag coefficient requires more energy to sustain a given velocity than one that passes through the air more easily. The field of aerodynamics is therefore of the utmost concern to automobile and airplane manufacturers as they attempt to build the most efficient vehicles.