A 175 m long train is traveling along a straight track with a velocity $72 \mathrm{~km} \mathrm{~h}^

Question

A 175 m long train is traveling along a straight track with a velocity $72 \mathrm{~km} \mathrm{~h}^{-1}$. A bird is flying parallel to the train in the opposite direction with a velocity $18 \mathrm{~km} \mathrm{~h}^{-1}$. The time taken by the bird to cross the train is

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(d) : Velocity of train, \ $ v_T=72 \mathrm{~km} / \mathrm{h}=72 	imes \frac{5}{18} \mathrm{~m} / \mathrm{s}=20 \mathrm{~m} / \mathrm{s} $ \ Velocity of bird, $v_B=18 \mathrm{~km} / \mathrm{h}=18 	imes \frac{5}{18} \mathrm{~m} / \mathrm{s}=5 \mathrm{~m} / \mathrm{s}$ Since bird is flying parallel to train in opposite direction. $	herefore \quad$ Relative velocity of bird w.r.t. train $=v_B+v_T=25 \mathrm{~m} / \mathrm{s}$ Train's length $=175 \mathrm{~m}$ Time taken by the bird to cross the train is $=\frac{175}{25}=7 \mathrm{~s}$