例重力的功d=Gc y =-mg(+y)甲 b -mge A ng mgyi-mgy y 例弹性力的功凵△A△m m F=-kxi 0 a X b A=[F·=- kxi dxi=-kxkx A=k例 重力的功 mgdy mgj dx i dyj dA G dr = − = −  + =  ( )      1 2 2 1 A m g dy mgy mgy yy = − = −  y 1 y2 a b y x dr  m G 例 弹性力的功 x 2 o x 1 b m x a m F x F kxi   = −    =  = −  = − 21 21 xx xx A F dx k xi dxi kxdx    22 21 21 21 A = k x − k x