R = fωv2
(1)
where f is a quantity approximately constant for any given surface. If
ξ = 2gf/G,
R = ξGωv2/2g,
(2)
where ξ is, like f, nearly constant for a given surface, and is termed the coefficient of friction.
The following are average values of the coefficient of friction for water, obtained from experiments on large plane surfaces, moved in an indefinitely large mass of water.
| Coefficient of Friction, ξ | Frictional Resistance in ℔ per sq. ft. f | |
| New well-painted iron plate | .00489 | .00473 |
| Painted and planed plank (Beaufoy) | .00350 | .00339 |
| Surface of iron ships (Rankine) | .00362 | .00351 |
| Varnished surface (Froude) | .00258 | .00250 |
| Fine sand surface (Froude) | .00418 | .00405 |
| Coarser sand surface (Froude) | .00503 | .00488 |