沿x方向的合力: F(x+dx, t)-F(x, *)=Sp(x+dr, t)-p(x, t)] Ou(x+dx, t)a =SY[(x+dx,1)-l(x,) ax 0l(x+x,1)-(x,t) dx]=sr dx dx 由牛顿第二定律:m=F(a=2,m=hv=s) 得 ll idx-=SY-dx at2 a2x ll 令a=,,并记: 有沿x方向的合力： 由牛顿第二定律： 得 令 ，并记： ，有 dx x u dx SY dx u x dx t u x t x SY u x dx t u x t x SY x u x u x dx t SY F x dx t F x t S p x dx t p x t 2 2 ] ( , ) ( , ) [ ] [ ( , ) ( , )] ( , ) [ ( , ) ( , ) [ ( , ) ( , )] ∂ ∂ = + − ∂ ∂ = + − ∂ ∂ = ∂ ∂ − ∂ ∂ + = + − = + − ( , ) 2 2 m dv sdx t u ma F a = ρ = ρ ∂∂ = = [ ( )] 2 2 2 2 2 2 x u x x u dx x u SY t u sdx ∂ ∂ ∂ ∂ = ∂ ∂ ∂ ∂ = ∂ ∂ ρ ρ Y a = 2 2 2 2 , x u u t u utt xx ∂ ∂ = ∂ ∂ = tt xx u a u 2 =