The drag force is dependent on the geometry and size of the object, the density of the air, the velocity of the object, and the so-called Reynolds number. In this implementation, the equation describing the drag force has been taken from the book Physics for Game Programmers, equation 7.51 on page 191, defined as:

• F_D = 1/2 ρ v^2 A C_D

where ρ is the air density, v is the velocity of the ball, A is the cross-sectional area of the ball, and C_D is the drag coefficient. The drag coefficient is dependent on the Reynolds number, which is in turn dependent on the air density, the velocity of the object, the cross-sectional area, and the viscosity. In Physics for Game Programmers, experimental data is used to describe C_D. This results in the following:

• C_D = 0.47 if Re< 100000
• C_D = 0.47 - 0.25*(Re -100000)/35000 if 100000 < Re < 135000
• C_D = 0.22 if Re > 135000

where Re is the Reynolds number.