Question

Find the initial velocity of a train which is stopped in 20 sec by applying breaks. The retardation due to breakes is 1.5 m/s-2

Answer 1

Final velocity (v)= 0m (as it comes to rest)
Acceleration (a)= -1.5m/s^2
Time (t)=20s
Let initial velocity be u
Using acceleration formula
a=(v-u)/t
-1.5 = (0-u)/20
-30 = -u
u=30
So, the initial velocity was 30m/s

Answer 2

Explanation:

The body(train) was moving with a velocity of 20m/s.

the driver must have put on brakes…such that it stops after 10 seconds.

so a deceleration or retardation due to brakes has reduced the speed to zero.

in one sec if the retardation is say a m/s^2 then in 10 sec it will reduce the velocity by 10 times a m/s^2

therefore

10 s. a m/s^2 = the initial velocity before the breaks was put on = 20 m/s

so a the retardation = 20/10 = 2m/s^2

or one can use kinematical relation

final velocity at time t say v = initial velocity - retardation. time

v(10sec) =0 = 20 m/s - a . 10 s

and one gets to the above result.

However if in another problem an acceleration is happen

then v(t) = u (t=0) + a.t

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