Air resistance creates a force that (for symmetric projectiles) is always directed against the direction of motion in the surrounding medium and has a magnitude that depends on the absolute speed: F a i r = − − f ( v ) ⋅ ⋅ v ^ ^ . The speed-dependence of the friction force is linear ( f ( v ) ∝ ∝ v ) at very low speeds (Stokes drag) and quadratic ( f ( v ) ∝ ∝ v 2 ) at larger speeds (Newton drag). The transition between these behaviours is determined by the Reynolds number, which depends on speed, object size and kinematic viscosity of the medium. For Reynolds numbers below about 1000, the dependence is linear, above it becomes quadratic. In air, which has a kinematic viscosity around 0.15 c m 2 / s , this means that the drag force becomes quadratic in v when the product of speed and diameter is more than about 0.015 m 2 / s , which is typically the case for projectiles.