解：$(1)G=mg=40\ \text {kg}×10\ \text {N/kg}=400\ \text {N}$
$ F_1·OD=G·OB$，代入数据，解得$F_{1}=200\ \text {N}$
$ (2)W=Fs=250\ \text {N}×2×0.4\ \text {m}=200\ \text {J}$
$ (3)W_{有}=Gh=400\ \text {N}×0.4\ \text {m}=160\ \text {J}$
$ η=\frac {W_{有}}{W}×100\%=\frac {160\ \text {J}}{200\ \text {J}}×100\%=80 \%$
