Question

a moving train is brought to rest in 20 sec by applying brakes. find the velocity with which the train was initially moving, if the ratardation due to brakes is 2m/s.s

Answer 1

40m/s.s is the answer.. 
Look if the train stopped in 20 seconds and the retardation was 2m/s.s 
So how much can be stopped in 20 seconds if 2m/s.s? 40m/s.s  
if the train kept moving then its velocity would have been greater than 40m/s.s but it eventually stopped in 20 seconds. So it had a velocity of 40m/s.s

Answer 2

$\bigstar\rm\blue{GIVEN}\bigstar$

• $\rm\blue{Final\:Velocity=0m/s^{-1}}$

• $\rm\red{Time\:Taken=20s.}$

• $\rm\red{Retardation=-2m/s^{-2}}$

$\bigstar\bf\red{TO\:FIND}\bigstar$

• $\huge\bf\red{Initial\:Velocity}$

$\bigstar\mathcal\red{FORMULAE\:USED}\bigstar$

• ${\boxed{\sf{\blue{V=U+at}}}}$

Using the above equation

$\implies\sf\blue{V=u+at}$

Where,

v=Final velocity

u=initial Velocity

a=Retardation

t=Time

Put the values in given equation

$\implies\bf\red{(0)=u+(-2\times{20})}$

$\implies\bf\blue{40m/s=U}$

Hence, The Train was moving with initial Velocity of $\mathbb\red{40m/s^{-1}}$.