$ 解：(1)W=Fs={ 200 }\ \text{N}×{ 2 }\ \text{m}={ 400 }\ \text{J}\\$
$ P=\frac {W}{t}=\frac {{ 400 }\ \text{J}}{{ 5 }\ \text{s}}={ 80 }\ \text{W}$
$ (2)W_有=Gh={ 30×10 }\ \text {N}×{ 1 }\ \text {m}={ 300 }\ \text {J}$
$ η=\frac {W_有}{W}×100\%=\frac {{ 300 }\ \text{J}}{{ 400 }\ \text{J}}×100\%={ 75 }\%$
$ (3)W_额=W-W_有=100\ \text {J}$
$ f=\frac {W_额}s=\frac {100\ \text {J}}{2\ \text {m}}=50\ \text {N}$
