$解：(1)η=\frac {G}{G+2G_动}=\frac {200\ \text {N}}{200\ \text {N}+2G_动}=50\%，$
$解得G_动=100{N}$
$(2)F_大=\frac {G'+2G_动}4=\frac {1600\ \text {N}+200\ \text {N}}4=450\ \text {N}$
$(3)η'=\frac {G'}{G'+2G_动}=\frac {1600\ \text {N}}{1600\ \text {N}+200\ \text {N}}≈88.9\%$
