3)弹性力作功 F=-kxi B B W= Fdx (A -kx4 lxx=0W kx x = − = d 0  F kxi = − 2 2 d d 1 1 ( ) 2 2 B B A A x x x x B A W F x kx x kx kx = = − = − −   A x B x F x o 3 ） 弹性力作功