革擦大寧 但博学求回新 South chi na Agricul tural Uni versity dp d = dy +",j+2k d l+ 十 k dt dt dt x 氵+a,j+ 2 +a,+a g cosa=x C0s阝= CoSy= 8D e p. p h y sic s D e p. p h y sic s D e p. p h y sic s 8 dt dv a    k dt dv j dt dv i dt dv a x y z        k dt d z j dt d y i dt d x    2 2 2 2 2 2    a x i a y j a z k       2 2 2 a  a  ax  ay  az cos a   ax cos a ay   a az cos   2 2 dt d r  