I'm writing a paper on how attaching non-conductive mass to a magnet and dropping it through a copper pipe to see how it affects the breaking force that acts on the magnet and therefore its terminal velocity as it exits the pipe. This is what I have so far: Breaking force = Force of Gravity, so mg=kv (m=mass, v=terminal velocity, and k is a constant). The constant k consists of several variables regarding the geometry and material composition of both the magnet and the pipe and a few experimentally and mathematically derived constants. My problem is that in the paper attached below which includes this formula for k, the denominator consists of the diameter to the fourth power. And in my investigation the pipe has around a 4mm larger radius than the one in the paper and because it is to the fourth power, there is a huge difference in our constants for k. The magnet used is the same dimensions and mass but could potentially be of different grades however that does not affect the constant significantly. There are a few other variables such as the correction parameter and the force factor that could potentially vary however I doubt it will to the point where can account for the 10x factor shrink of my constant compared to my experimentally derived constant. (Yes I ran an experiment so if you are genuinely interested in helping me then I can provide you with what I have so far.) Anyway, if you have any idea if something else can correct this error then please let me know!! That's all. :) Look at page 3 and equation 10 in this: Donoso Paper .