Question

A person standing on a railway platform noticed that a train took 21 seconds to completely pass through platform which was 84 m long and it took 9 seconds in passing him. the speed of the train was?

Answer 1

Let l be the length of the train and v the velocity of the train.

Then :

v = l / 9 in regards to the train passing the man.

v = (l + 84)/21 when it passes through the platform.

We equate the two :

l/9 = (l + 84) / 21

21l = 9(l + 84)

21l = 9l + 756

21l - 9l = 756

12l = 756

l = 63

Now substituting this in l/9 we have :

v = 63/9 = 7

= 7m/s