I was pleased to read Pirooz Mohazzabi’s recent article,1 which discusses air drag in projectile motion experiments and describes the drag force on a projectile as FD = cv2. The constant c, which has units of kg/m, has a physical significance—it can be interpreted as the mass of the air volume “pushed aside” for every meter traveled by the object. This can be seen more clearly from the full expression for drag force,2 


where cD is a dimensionless drag coefficient ~1 that depends on the object’s shape, A is the object’s frontal area, and ρair is the air density, assumed to be 1.22 kg/m3 in the laboratory.

An intuitive way to parameterize the effects of drag is the object’s terminal speed...

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