So I did a project about a year ago and I cant remember one of the things I wrote in this equation. The equation apparently I got it from NASA's website. https://www.grc.nasa.gov/www/k-12/airplane/dragco.html This is my equation CD = FL/ (0.5 * Air viscosity * V^2 * Area) = 0.4 These are the numbers I used to plug in the equation Velocity = 13 m/s Diameter pipe = 0.000635 m Air viscosity = 0.00001599 kg/m*s FL = 1*10^-9 Kg <<< this is the problem Area of Dust = 1.96 * 10^-11 Microns ____________________________ Now in NASA's website instead of FL there's a "D" and it says the "D" represents the drag. Now I can't remember what (FL) means, is it truly a drag? and a drag of what exactly. My project was about compressed air moving inside a small tube, and the tube has pinched hols in it to pump the compressed air to clean a surface from dust. SO, can someone tell me what the "FL" that I wrote means? and what exactly does the number FL = 1*10^-9 Kg represent? Could it be "air drag??" I am not sure where I found this number.