where ⁠ a ⁠ is the radius of the spherical particle, ⁠ n ⁠ is the number density of gas species, ⁠ m ⁠ is their mass, c ¯ ¯ is the arithmetic mean speed of gas species, and ⁠ u ⁠ is the relative speed of the sphere with respect to the rest frame of the gas. The factor δ δ encompasses the microphysics of the gas-sphere interaction and the resultant distribution of velocities of the reflected particles, which is not a trivial problem. It is not uncommon to assume δ δ = 1 (see below) presumably in part because empirically δ δ is found to be close to 1 numerically, and in part because in many applications, the uncertainty due to δ δ is dwarfed by other uncertainties in the problem. For this reason, one sometimes encounters Epstein drag written with the factor δ δ left absent. The force acts in a direction opposite to the direction of motion of the sphere. Forces acting normal to the direction of motion are known as "lift", not "drag", and in any case are not present in the stated problem when the sphere is not rotating.