Given, length of train, l=100m Time taken by an object to cross the train, t=7.2s Velocity of object opposite to train, v0=5‌km‌h−1=
25
18
m∕s=1.39m∕s Let the velocity of train, vt=v Using the concept of relative motion. Relative velocity of train w.r.t object vrel=v1+v0 Now, l=vrel×t 100=7.2(vt+v0) ⇒100=7.2(v+1.39) ⇒v=12.5m∕s=45‌km‌h−1