Question 43373
from the viewpoint of the man on the slower train, he is stationary and the faster train is going at 18kmph. This is the basics of "relativity".


Anyway, we now have speed and time. We can therefore find the length.


First we need all numbers to have consistent units. We have speed in kmph and a time in seconds. We need to convert kmph into km per second OR convert the 15 seconds into hours. The second is easier


Start with something you know:
60 seconds = 1 minute
so 1 second = 1/60 minutes
and hence 15 seconds = 15/60 minutes


so, 15 seconds = 1/4 minute


Again, start with something we know:
60 minutes = 1 hour
so 1 minute = 1/60 hours
and hence 1/4 minutes = (1/4)*(1/60) hours


so 15 seconds = 1/240 hours


speed = distance/time
so distance = speed*time
--> distance = 18*(1/240)
distance = 0.075km


and converting this to something more meaningful, we have distance = 75m


So, length of train is 75m long.


Jon.