Terminal velocity is the term for the state an object reaches when the force of drag acting on it is equal to the force of gravity acting on it. When an object reaches its terminal velocity, it no longer accelerates, remaining at whatever velocity it was already traveling or else slowing down.

As an object accelerates, the amount of drag exerted on it increases. This means that more force is necessary to sustain the same level of acceleration. If that external force is increasing, as in a car or plane, then the object can be accelerated well past its terminal velocity. If, however, the only force being exerted on it is the force of gravity, then eventually the drag will become as great as the static force of gravity, and the object will cease to accelerate.

An object may also decelerate towards terminal velocity, if it was initially moving faster than terminal velocity. This may be because it entered from somewhere with less drag, such as the thinner upper atmosphere, or because it was initially launched with some external force other than gravity at a greater velocity. In this way, terminal velocity can be viewed as a sort of equilibrium point that objects in freefall naturally gravitate towards.

Strictly speaking, an object never actually reaches its terminal velocity, it simply reaches a state which approximates it. Instead, in a manner similar to Zeno’s paradox of motion, the object comes closer and closer to its terminal velocity, reducing its acceleration to miniscule amounts, until the acceleration is no longer even measurable or functional, and terminal velocity is said to have been achieved.

Different objects will have drastically different terminal velocities. A person in freefall, for example, has a terminal velocity of approximately 184 ft./s (56m/s) or roughly 124mph (200kph). A raindrop, in contrast, although very aerodynamic, is also not very dense, and so has a terminal velocity of around 25 ft./s (82m/s) or roughly 17mph (27kph). A lead bullet shot up straight in the air, on the other hand, has a terminal velocity of around 223 ft./s (68m/s) or 152mph (245kph).

Calculating the terminal velocity of an object means calculating the drag exerted on the object, and then comparing that to the weight of the object to determine the net force. To do this, you need to know the frontal area of the object, the gas density it is falling through, and a drag coefficient. It’s not a particularly easy calculation to make, although there are online calculators which can help you make rough estimates as to an object’s terminal velocity. The basic rules of thumb, though, are that objects will have a higher terminal velocity when: there is more of it, making a heavier weight; the density of the gas it’s falling through is lower, as in upper atmosphere; the object has a lower drag coefficient, meaning it has been built to be more streamlined, like a raindrop; or there is less area to drag, usually meaning the object is more dense.