$解：(1)h=2×3\ \text {m}=6\ \text {m}$
$W_有=Gh={ 350 }\ \text {N}×{ 6 }\ \text {m}={ 2100 }\ \text {J}$
$W_总=Fs={ 200 }\ \text{N}×{ 2×6 }\ \text{m}={ 2400 }\ \text{J}\\$
$W_额=W_总-W_有=2400\ \text {J}-2100\ \text {J}=300\ \text {J}$
$(2)P_总=\frac {W_总}{t}=\frac {{ 2400 }\ \text{J}}{{ 120 }\ \text{s}}={ 20 }\ \text{W}$
$(3)η=\frac {W_有}{W_总}×100\%=\frac {{ 2100 }\ \text{J}}{{ 2400 }\ \text{J}}×100\%={ 87.5 }\%$
