A dimensionless number used in momentum transfer in general and free settling velocities and resistance to flow calculations in particular. Proportional to (Gravitational force) / (Inertial force).

C_{D} = (g (rho-rho_{f}) L) / (rho V^{2})

g: gravitational acceleration, L: characteristic dimension of object, rho: density of object, rho_{f}: density of surrounding fluid, V: velocity

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