1. ## Finding length

I have a question which i am unable to understand.

Question:A passenger train traveled at a speed of 72 km/h. A man on the passenger train observed a goods train travelling at a speed of 54 km/h in the opposite direction. If the goods train passed him in 8 seconds, find the length of goods train.

Please explain to me how should this question be solved as i have another question like this to solve.

2. ## Re: Finding length

Start by drawing a picture of the situation...

3. ## Re: Finding length

First find relative speed of train = 72 + 54 = 126 km/h
now convert km/h to meters/sec, which would be 1000/3600
so relative speed of train = 126 x (1000/36) m/s
which is 35 m/s

we know that slower train passes man in faster train in 8 seconds, so the length of the slower train = distance it covers in 8 seconds
so that is 35 x 8 = 280 meters

I think that's right anyway. It's quite a common type of time/speed/distance question.

4. ## Re: Finding length

Originally Posted by Mathstutorjim
First find relative speed of train = 72 + 54 = 126 km/h
now convert km/h to meters/sec, which would be 1000/3600
so relative speed of train = 126 x (1000/36) m/s
which is 35 m/s

we know that slower train passes man in faster train in 8 seconds, so the length of the slower train = distance it covers in 8 seconds
so that is 35 x 8 = 280 meters

I think that's right anyway. It's quite a common type of time/speed/distance question.
126 x (1000/36) does not equal 35 m/s, make sure the units are correct, the correct answer is (126*1000)/(60*60) = 126000/3600 = 35 m/s