Terminal VelocityWhen an object which is falling under the influence of gravity or subject to some other constant driving force is subject to a resistance or drag force which increases with velocity, it will ultimately reach a maximum velocity where the drag force equals the driving force. This final, constant velocity of motion is called a "terminal velocity", a terminology made popular by skydivers. For objects moving through a fluid at low speeds so that turbulence is not a major factor, the terminal velocity is determined by viscous drag. The expression for the terminal velocity is of the form
Objects moving at high speeds through air encounter air drag proportional to the square of the velocity. This quadratic drag leads to a terminal velocity of the form

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Quadratic Velocity DependenceFor large objects moving through air, the air resistance is approximately proportional to the square of the velocity. The form of the resistance is

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Terminal Velocity Examples
Data from Serway, Physics for Scientists and Engineers, Table 6.1. A drag coefficient C=0.5 is assumed, falling through air. Hailstone calculationCalculation for a sphere 
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Hailstone Terminal VelocityContributing to the danger of large hailstones is the fact that they fall faster than small ones. That is, the terminal velocity increases with the size of the hailstone. Assuming the hailstones to be spherical and using a drag coefficient of C=0.5 gives the following :

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Terminal Velocity CalculationFor a spherical object falling through air, the terminal velocity for the case of quadratic drag can be calculated from

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