Question

A train slows down from 80 km/h with a
uniform retardation of 2 m/s. How long will it take to
attain a speed of 20kmh '? ​

Answer 1

Explanation:

You mean how long before it stops?

0=80000/3600–2*t

0=(8 x 10^4)/(36 x 10^2)-2*t

0= 8/36 x 100–2*t

22.2=2*t

t=11.1 s

Answer 2

Assuming constant force:  

vf0t=vi+at>80⋅1000602−2t>1009⪆11 s