What is the coefficient of drag, Cd, of a tennis ball (roughly d = 6,5 cm, m = 57 g)? Generally; where do I find such coefficients? I'd like to find coefficients for projectiles in water too - since these are supposedly not the same, because of the water viscosity. I've found the "Drag Coefficient" text on Scienceworld Wolfram, which apparently equals Cd to Re^(-1/2), where Re is the Reynolds number. But the equation from which the text gets Cd has L in it, the "size scale" of the body - which is squared. But in my equation there is no such scale, only the silhouette area A of the body, which isn't squared. Why this difference? Also - the "length scale" l is in the Cd equation. What is it? I'm quite puzzled. If the Cd can be found directly from a Reynolds number, which I'm not sure it can, could my problem then be solved?