Question

a moving train is stopped by brakes in 20 s. find u if acceleration is -2m/s^2

Answer 1

time =20sec
acc=-2m/s^2  [retardation]
final velocity   v  =0m/s^2
 v-u/t=a
0-u/20=-2
-u=-2*20
u=40m/s^2

Answer 2

V=0m/s as it stops
a=-2m/s²
t=20 sec
u=?
Putting these values in 1st eq of motion
V=u + at
0=u + (-2)20
0=u +(-40)
40 = u