in order to solve these problems it might be best to deal with the distances between points a, b, c and d as being all starting at a ... by choosing this tactic we can use a constant value for u
Usually when 2 cars/trains/donkeys are passing each other they usually catch up, one is going faster but the other one has the better acceleration .....
they meet when the distances from a common start point are equal, if not common make the adjustments
they are furthest apart when they have the same velocity, the highest a ball is in the sky is when Vy = 0 thats also cos the Vground = 0,
@ point of maximum distance between the particles Va = Vb
Another way to do this is to find the equation of the distance between the bodies S1 - S2 and differentiate this wrt t