Answer:
\[{{m}_{1}}=400\,\,kg\,\,{{u}_{1}}=72\,km/h=20m/s\] \[{{m}_{2}}=4000\,\,kg\] \[{{u}_{2}}=9km/h=2.5m/s\] \[{{v}_{1}}=?,\,\,{{v}_{2}}=18km/h=5m/s\] By law of conservation of linear momentum, \[{{m}_{1}}{{u}_{1}}+{{m}_{2}}{{u}_{2}}=-{{m}_{1}}{{v}_{1}}+{{m}_{2}}{{v}_{2}}\] \[\Rightarrow 400\times 20+4000\times 2.5=-400\times {{v}_{1}}4000\times 5\] \[\Rightarrow 8000+10000=-400\,\,{{v}_{1}}+20000\] \[\Rightarrow 400{{v}_{1}}=20000-18000\] \[\Rightarrow {{v}_{1}}=5\,\,m{{s}^{-1}}=5\times \frac{18}{5}=18km/h\]
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