Air resistance is drag produced by motion relative to air. It transfers momentum between the body and the air and can change both speed and trajectory. The relevant speed is relative to the local velocity field, so a wind changes the force even when the body's velocity relative to the ground is unchanged.
A quadratic model writes , with specified reference area . It is a regime-dependent representation, not a universal formula with constant . For a falling object under steady downward motion, terminal velocity occurs when the upward drag and any buoyancy balance the object's weight. Comparing differently shaped bodies of equal mass illustrates why neglecting air resistance can be accurate for one object and poor for another, even over the same fall distance.
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