Air resistance

Meaning: Air resistance is the drag force air exerts on an object moving through it, directed opposite the object's velocity relative to the air. It increases with speed, cross-sectional area and air density. A falling object therefore accelerates more slowly as it speeds up, until drag equals weight and it falls at constant terminal velocity. Drag is nonconservative: it converts mechanical energy into thermal energy.

Equation: Fdrag = ½ρCAv², a common model for fast objects, where Fdrag is the drag force (N), ρ is air density (kg/m³), C is the dimensionless drag coefficient, A is cross-sectional area (m²), and v is speed relative to the air (m/s). At terminal velocity, mg = Fdrag.

On the MCAT: Skydiver or falling-object passages ask for a velocity–time graph that levels off at terminal velocity, or for the effect of opening a parachute (larger A lowers terminal velocity). Problems that say to neglect air resistance use ideal kinematics, where only gravity acts.

Common confusion: Terminal velocity does not mean the object has stopped moving. Net force is zero, so acceleration is zero while speed stays constant. Also, with no air, all objects fall with the same acceleration g. Differences in how fast objects fall in air come from drag, not from gravity itself.