= Air resistance
<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 $\mathbf F=-\tfrac12 C_D\rho A|\mathbf v_{\rm rel}|\mathbf v_{\rm rel}$, with specified reference <area> $A$. It is a regime-dependent representation, not a universal formula with constant $C_D$. 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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