Question

a boy moving by a train with a velocity of 50km/sec accelerate for 10 sec at the rate of 20 km/sec. find its final velocity. ​

Answer 1

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$\huge \underline \red {\sf{ Solution :-}}$

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 = $\small \underline {\sf{20/10 = 2m/s^2 }}$

Or one can use kinematical relation :-

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

$\small \underline {\sf{ v(10sec) = 0 = 20 m/s - a . 10 s}}$

and one gets to the above result.

However if in another problem an acceleration is happen

then $\small \underline {\sf{v(t) = u (t = 0) + a.t }}$

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Answer 2

Answer:

v = u + at

v = 50 + 200

v = 250 km/sec