Terminal velocity

Terminal velocity is the steady speed an object reaches while moving through a fluid when drag and buoyancy balance its weight. At that speed, its acceleration is zero.

Drag equation: An equation relating fluid drag to density, speed, drag coefficient, and reference area. Its speed-squared form often determines terminal velocity at high Reynolds numbers.
Newton's second law: The law stating that an object's net force equals its mass times its acceleration. Terminal velocity follows when the net force and therefore acceleration become zero.
Skydiving: The sport of falling through the atmosphere before deploying a parachute. A skydiver approaches one terminal speed before opening the parachute and a lower one afterward.
Terminal settling velocity: The steady downward speed of a particle settling through a fluid under gravity. This specialized usage applies terminal-velocity reasoning to particles in suspensions.
Stokes' law: A relation giving the viscous drag on a small sphere moving slowly through a fluid. It predicts terminal speed for tiny spheres in the low-Reynolds-number regime.
Gravitational force: The attractive force exerted by gravity between objects with mass. Near Earth's surface, it supplies the object's downward weight.
Parachute: A fabric device that increases drag to slow a falling person or object. Its large area sharply lowers the terminal speed of a falling skydiver.
Escape velocity: The minimum speed an object needs to escape a celestial body's gravitational field without further propulsion. It is a threshold speed for escape, not a steady speed set by fluid forces.
Buoyancy: The upward force a fluid exerts on an immersed object, equal to the weight of displaced fluid. It reduces the effective downward force that drag must balance.
Weight: The gravitational force acting on an object. It is the downward force balanced by drag and buoyancy at terminal velocity.

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