解：?$(1)F_{乙}=G-F_{甲}=120\ \text {N}-80\ \text {N}=40\ \text {N}$?
?$F_{甲}l_{甲}=F_{乙}l_{乙}①$?
?$l_{甲}+l_{乙}=1.8\ \text {m}②$?
聯(lián)立①②，解得?$l_{甲}=0.6\ \text {m}$?
?$ (2)l_{甲}'=0.6\ \text {m}+0.2\ \text {m}=0.8\ \text {m}$?，?$l_{乙}'=1.8\ \text {m}-0.8\ \text {m}=1\ \text {m}$?
?$ F_{甲}'×1.8\ \text {m}=120\ \text {N}×1\ \text {m}$?
?$ $?解得?$F_{甲}'≈66.7\ \text {N}$?
?$ $?減小了?$?F_{甲}=80\ \text {N}-66.7\ \text {N}=13.3\ \text {N}$?
