Physics question please help?

Question

A train is traveling down a straight track at 21 m/s when the engineer applies the brakes, resulting in an acceleration of -1.0 m/s2 as long as the train is in motion. How far does the train move during a 39 s time interval starting at the instant the brakes are applied?

Answer 1

Sorry I did not read the whole problem the train is completely stopped after 21 s, so it does not go any further

Given: v0= 21 m/s, a= - 1.0 m/s^2, t =39 s, x0=0m

Find: t for how long the train was moving

v0= initial velocity, a=acceleration, t=time, x0=starting point, xf=distance traveled, ^2=squared

vf=v0+at

0 m/s=21 m/s +(-1.0 m/s^2)(t)

-21 m/s=(-1.0 m/s^2)(t)

(-21 m/s) / (-1.0 m/s^2) =t

21 s = t

Given:v0= 21 m/s, a= - 1.0 m/s^2, t =21 s, x0=0m

Find: xf

xf=x0+v0t-(1/2)(a)(t^2)

xf=0m+(21 m/s)(21s)-(1/2)(-1.0 m/s^2)(21 s)^2

xf=0 m + 441 m - (-0.50 m/s^2)(220.5 s^2)

xf=441 m - (-220.5 m)

xf=441 m + 220.5 m

xf= 661.5 m