Final answer:
The initial velocity of train A before applying the brakes and coming to a stop just as train B collided with it was approximately 28.28 m/s.
Step-by-step explanation:
To determine the initial velocity of train A, we can use the kinematic equation for uniformly accelerated motion:
v2 = u2 + 2as
Where v is the final velocity (0 m/s since train A stops), u is the initial velocity (what we're trying to find), a is the acceleration (-1 m/s2 because the train is decelerating), and s is the distance (400 meters).
Rearranging the formula to solve for u gives:
u = sqrt(v2 - 2as)
= sqrt(0 - 2(-1)(400))
= sqrt(800)
= 28.28 m/s (approximately)
Therefore, the initial velocity of train A was approximately 28.28 m/s.