A model rocket blasts off from the ground, rising straight upward with a constant acceleration that has a magnitude of 76.0 m/s2 for 1.90 seconds, at which point its fuel abruptly runs out. Air resistance has no effect on its flight. What maximum altitude (above the ground) will the rocket reach?

Answer 1

After t = 1.9 seconds, it attains a height of
H = (a/2) t^2 = 137.2 m
and a velocity of
V = a*t = 144.4 m/s

As the kinetic energy is converted to gravitational energy at maximum altitude, it will rise an additional distance H' given by
g*H' = V^2/2
H' = 1064 m

The maximum altitude is H + H'

Answer 2

I still am not understanding how to obtain the answer