Editing Terminal velocity (section)

{{Short description|Highest velocity attainable by a falling object}}
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{{More citations needed|article|date=March 2012}}
[[File:Terminal velocity.svg|thumb|upright=0.7|The downward force of gravity (''F<sub>g</sub>'') equals the restraining force of drag (''F<sub>d</sub>'') plus the buoyancy. The net force on the object is zero, and the result is that the velocity of the object remains constant.]]

'''Terminal velocity''' is the maximum speed attainable by an object as it falls through a [[fluid]] ([[air]] is the most common example). It is reached when the sum of the [[Drag (physics)|drag]] force (''F<sub>d</sub>'') and the [[buoyancy]] is equal to the downward force of [[gravity]] (''F<sub>G</sub>'') acting on the object. Since the [[net force]] on the object is zero, the object has zero [[acceleration]].<ref>{{Cite web |title=6.4 Drag Force and Terminal Speed - University Physics Volume 1 {{!}} OpenStax |url=https://openstax.org/books/university-physics-volume-1/pages/6-4-drag-force-and-terminal-speed |access-date=2023-07-15 |website=openstax.org |date=19 September 2016 |language=en}}</ref><ref>{{cite journal |last1=Riazi |first1=A. |last2=Türker |first2=U. | date=January 2019 |title=The drag coefficient and settling velocity of natural sediment particles |journal=Computational Particle Mechanics |volume=6 |issue=3 |pages=427–437 | doi=10.1007/s40571-019-00223-6|bibcode=2019CPM.....6..427R |s2cid=127789299 }}</ref> For objects falling through air at normal pressure, the buoyant force is usually dismissed and not taken into account, as its effects are negligible.

As the speed of an object increases, so does the drag force acting on it, which also depends on the substance it is passing through (for example air or water). At some speed, the drag or force of resistance will be equal to the gravitational pull on the object. At this point the object stops accelerating and continues falling at a constant speed called the terminal velocity (also called '''settling velocity'''). 

An object moving downward faster than the terminal velocity (for example because it was thrown downwards, it fell from a thinner part of the atmosphere, or it changed shape) will slow down until it reaches the terminal velocity. Drag depends on the [[projected area]], here represented by the object's cross-section or silhouette in a horizontal plane.

An object with a large projected area relative to its mass, such as a parachute, has a lower terminal velocity than one with a small projected area relative to its mass, such as a dart. In general, for the same shape and material, the terminal velocity of an object increases with size. This is because the downward force (weight) is proportional to the cube of the linear dimension, but the air resistance is approximately proportional to the cross-section area which increases only as the square of the linear dimension. 

For very small objects such as dust and mist, the terminal velocity is easily overcome by convection currents which can prevent them from reaching the ground at all, and hence they can stay suspended in the air for indefinite periods. Air pollution and fog are examples.