# Stoke's Law (drag force)

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F_(drag) =
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Stoke's Law (drag force) computes the force of drag based on the dynamic viscosity (mu) of the fluid (e.g. water or air), the radius of the spherical object (R), and the velocity (V).
The results are the force in Newtons.

## Dynamic Viscosity

The dynamic viscosity of air depends mostly on the temperature.
At 15 °C, the viscosity of air is 1.81 x 10-5 kg/(m·s), 18.1 μPa.s or 1.81x10-5 Pa.s.

## Applications

Stoke's law and equation has been found useful in low velocity (under 55 mph) environments such as the flight of a football.  Note: a football is 6.8" across, and hence has a radius (R) of 3.4".