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Drop a bowling ball and a feather in a vacuum chamber, and they hit the floor together at full acceleration. Drop them through the air outside, and the feather floats gently down while the bowling ball crashes down fast.
Air isn't empty space β€” it's a sea of gas molecules that push back against anything moving through it with a velocity-dependent drag force.
How exactly does this opposing force grow as an object speeds up?

The Two Regimes of Drag

When an object moves through a fluid, the resisting force depends on how fast the object travels and whether the fluid flows smoothly or churns turbulently.
For tiny objects moving slowly through thick fluids (like a dust grain in air or a bead in syrup), the flow is smooth and linear drag applies: Fd=bvF_d = bv, where bb is a constant based on fluid viscosity and object size.
In 1851, physicist George Gabriel Stokes derived this linear relationship while calculating how pendulums slow down in fluids.
For everyday human-scale objects moving at normal speeds, the fluid becomes turbulent, and quadratic drag dominates: Fd=12CdρAv2F_d = \frac{1}{2} C_d \rho A v^2.
As the falling object accelerates, drag increases until something dramatic happens to the net force. What is that state?

Reaching Terminal Velocity

According to Newton's second law, net force dictates acceleration: Ξ£F=mgβˆ’Fd=ma\Sigma F = mg - F_d = ma, choosing downward as the positive direction.