Andy C. answered • 07/30/17

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4.5 km/hr * 1 hr/60 min * 1 min/60 sec * 1000 m/ 1 km = 4500/3600 m/sec = 45/36 m/sec = 5/4 m/sec = 1.25 meters per sec

5.4 km/hr * 1 hr/60 min * 1 min/60 sec * 1000 m/ 1 km = 5400/3600 m/sec = 54/36 m/sec = 3/2 m/sec = 1.5 meters per sec

In 8.4 seconds, the distance between the two persons is 8.4 * ( 1.5m - 1.25m) = 8.4 x .25 = 2.1 meters

It took the train 0.1 second to travel 2.1 meters.

That's 21 meters per second.

21 m/sec * 60 sec/ 1 min * 60 min/1 hour * 1 km/1000 m = 75.6 km/hour

Rakesh P.

but you have done it wrong. the train will have some length of its own. that also needs to be added in the ttal distance travelled by tarin

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07/30/17

Andy C.

Yes, but it gets cancelled out by the distance that the two people walked.

Moreover, we know NONE of these distances walked by the people or traveled by the train.

The only thing we have is the 2.1 meters between the two pedestrians and the 0.1 seconds

of time the train takes to pass them both. That is all that is needed to find the speed of the train.

Try to model the problem like this

------------------------------------------------------Xr=4.5

----------------------------------------------------------------------Xr=5.4

where Xr marks the spot of the pedestrians as the train passes by.

In that short time-span of 0.1 second, the train travels 2.1 meters.

Distance = rate x time.

DIstance = 2.1 meters

Time = 0.1 seconds.

That's all we need to find the speed of the train.

What happens before that is irrelevant.

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07/30/17

Rakesh P.

07/30/17