Old English hit , neuter nominative and accusative of third person singular pronoun, from Proto-Germanic demonstrative base *khi- (cf. Old Frisian hit , Dutch het , Gothic hita "it"), from PIE *ko- "this" (see he ). Used in place of any neuter noun, hence, as gender faded in Middle English, it took on the meaning "thing or animal spoken about before."
The h- was lost due to being in an unemphasized position, as in modern speech the h- in "give it to him," "ask her," "is only heard in the careful speech of the partially educated" [Weekley]. It "the sex act" is from 1610s; meaning "sex appeal (especially in a woman)" first attested 1904 in works of Rudyard Kipling, popularized 1927 as title of a book by Elinor Glyn, and by application of It Girl to silent-film star Clara Bow (1905-1965). In children's games, meaning "the one who must tag the others" is attested from 1842.
Forces that can change the momentum of a droplet include the gradient of the pressure and gravity, as above. In addition, surface forces can deform the droplet. In the simplest case, a shear stress τ , exerted by a force parallel to the surface of the droplet, is proportional to the rate of deformation or strain rate . Such a shear stress occurs if the fluid has a velocity gradient because the fluid is moving faster on one side than another. If the speed in the x direction varies with z , the tangential force in direction x per unit area normal to the z direction is