Terminal velocity: parachutes, raindrops and top speeds

A raindrop that fell $1\ \text{km}$ with no air would land at about $140\ \text{m s}^{-1}$. Every rain shower would be a hail of bullets.

Real big raindrops land at about $9\ \text{m s}^{-1}$, because they reach terminal velocity long before they hit you.

Terminal velocity isn't only about falling. It happens whenever a constant driving force is opposed by a resistive force that grows with speed:

• a falling object: driving force = weight;
• a car with the accelerator pressed to the floor: driving force from the engine, opposed by friction and air resistance;
• a ball bearing sinking through oil: weight, opposed by viscous drag (and upthrust).

In each case the object speeds up until the resistive force has grown enough to balance the driving force. Resultant force zero, acceleration zero, constant velocity: the top speed.