瞬时速率: S =Iim△s_ds 0△tdt 0 r(计+△) 瞬时速度和瞬时速率的关系: d ds dr dt dt ds ;=dF/(d切向单位矢量 12 1+p 1313 t s t s v t d d = = →0 lim 瞬时速率: 2 2 2 v = v = v x + vy + vz t dr ds—切向单位矢量 e ˆ = r s 0 r(t+t) r(t) t e ˆ t r v d d = 瞬时速度和瞬时速率的关系: t = v e ˆ s r t s d d d d =