Aerodynamics is the study of air in motion, which includes changes in the physical characteristics, such as pressure and density. (Thermodynamics is similar but is likely to involve significant temperature changes). Because the air is in motion, changes in velocity and mass flow-rates are also important. Aerodynamics also involves the study of forces being generated (e.g. the "lift" force on a wing), and so a brief mention must be made of some basic principles.