解：$(1)Q_{放}=mq=1350\ \text {kg}×4×10^{7}\ \text {J/kg}=5.4×10^{10}\ \text {J}$
$ (2)v=270\ \text {km/h}=75\ \text {m/s}$
$ F=\frac{P}{v}=\frac{2.25×10^{7}\ \text {W}}{75\ \text {m/s}}=3×10^{5}\ \text {N}$
$ f=F=3×10^{5}\ \text {N}$
$ (3)W=Pt=2.25×10^{7}\ \text {W}×20×60\ \text {s}=2.7×10^{10}\ \text {J}$
$ η=\frac {W}{Q_{放}}×100\%=\frac {2.7×10^{10}\ \text {J}}{5.4×10^{10}\ \text {J}}×100\%=50 \%$
