A 120m long train is moving towards west at a speed of 10m/s. A small bird flying towards east with a speed of 5m/s crosses the train. Find the time taken by the bird to cross the train in second???

The relative velocity of train with respect to bird is 10+5=15 m/s

Hence in a frame of reference in which bird is at rest the train is moving with velocity 5m/s

The train is 120m long hence time taken for train to pass (from where bird is) end to end is

speed=distance/time

time=distance/speed

time=120/15

time=8 sec

Hence bird takes 8 sec to pass the train