2) The force of kinetic friction(动摩擦力, (滑动)) (5-8 Where uk is the coefficient of kinetic friction Usually, for a given pair of surfaces us >Lp The actual value of us and uk depend on the nature of both the surfaces in contact➢ 2)The force of kinetic friction(动摩擦力, （滑动）): (5-8) where is the coefficient of kinetic friction. f k = k N k k  s   k  s Usually, for a given pair of surfaces . The actual value of and depend on the nature of both the surfaces in contact