Energy in transport

300px|thumb|upright=1.6|A NASA wake turbulence study at Wallops Island in 1990. A [[vortex is created by passage of an aircraft wing, revealed by smoke. Vortices are one of the many phenomena associated with the study of aerodynamics.]]

force resisting the motion when a body (such as a ball, tire, or wheel) rolls on a surface

unmotorised transport powered by human physical activity