Any object moving through space, above the ground and within our atmosphere can be thought of as being in flight. Birds, planes, helicopters, Frisbees, and baseballs, if moving through the air, are all subject to the physics of flight. The physics of flight, or aeronautics, is similar to other branches of physics in that it is all about the balancing of forces. A body in straight and level flight can be said to be in equilibrium.

Four forces act upon a body in flight; two forces aid flight and two oppose it. Forward momentum is provided by thrust. Upward movement, away from the ground, is provided by lift. Air resistance is called drag, and the tendency of an object to fall toward Earth is gravity.

Thrust is the motive power that propels a flying object forward. An airplane's engine, a baseball pitcher's arm, a...