Note that there are two approaches to calculating v in this artcile, and this derivation covers only the more hand-wavy one. For example, they drop the √f facto

Note that there are two approaches to calculating v in this artcile, and this derivation covers only the more hand-wavy one. For example, they drop the √f factor, because "[f]or most strongly bonded solids, f varies in the range of 1 to 4".