Apply the concept of relative velocity

[winnie3]

I cannot understand how to apply the concept of relative velocity in certaijn questions like the below one
the question is Two trains A and B of length 400m each are moving on two parallel tracks with a uniform speed of 72km/hr in the same direction with A ahead of B The driver of B decides to overtake A and accelerates byt 1m/s2 If after 50s the guard of B just brushes past the driver of A, what was the original distance between them?
sir, I know that train B's acceleration is 1m/s^2 time is 50s initial velocity is O and by equations of motion we can find the distance. But my teacher said here the concept of relative velocity comes into play. Please explain me why and how?

 

[Gib Z]

This shouldn't be in the Tutorials section. This section is for knowledgeable people to post tutorials.

To your question,

We can take them to both be going at v=0 yes? Relative to each other.

O btw, a is constant, 1, we can use the equations now. s=ut + (at^2)/2. Let a=1 and make other relavent substitutions.

Moderator, move this thread please.

Edit: I just realized i sounded really cold in that post, no disrespect winnie3, hope u have fun here at physicsforums.com :)