$解：(1)W=Pt={ 80 }\ \text {W}×{ 50 }\ \text {s}={ 4000 }\ \text {J}$
$F=\frac WL=\frac {4000\ \text {J}}{10\ \text {m}}=400\ \text {N}$
$(2)W_有=Gh={ 1200 }\ \text {N}×{ 2 }\ \text {m}={ 2400 }\ \text {J}$
$η=\frac {W_有}{W}×100\%=\frac {{ 2400 }\ \text{J}}{{ 4000 }\ \text{J}}×100\%={ 60 }\%$
$(3)W_額=W-W_有=4000\ \text {J}-2400\ \text {J}=1600\ \text {J}$
$f=\frac {W_額}{L}=\frac {1600\ \text {J}}{10\ \text {m}}=160\ \text {N}$
