Question

# Two trains are moving on parallel tracks in opposite directions at the speeds of 20 m/s and 15 m/s respectively. If both the trains are of equal length 140 m each, then how much time will they take to cross each other?

A
4 s
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B
8 s
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C
16 s
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D
2 s
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Solution

## The correct option is B 8 sAs both trains are moving in opposite direction the relative velocity of train 1 with respect to train 2 will be: ⇒vrel=v12=v1−(−v2)=v1+v2 ⇒v12=20+15=35 m/s Here, the displacement of train (1) from frame of reference of train (2) , when both trains cross each other will be equal to sum of length of both trains: ⇒l=l1+l2 Or, l=140+140 ∴l=280 m ∴ Total time required to cross each other : ⇒t=lv12=l1+l2v12 ∴t=28035=8 s.

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