posted by james .
A train is speeding along a straight, level track at +20.0 m/s when the engineer applies the brakes, which results in an acceleration of -0.894 m/s2 as long as the train is in motion. How far does the train move during the 70 sec after the brakes are first applied?
Key phrase is “as long as the train is in motion”: it moves during the time interval t<70 s.
The train stops after t seconds of braking
t=(v-vₒ)/a=( 0-20)/(-0.894)=22.4 s.
s= vₒt-at²/2 =20•22.4 – 0.894•22.4²/2=223.7 m