Question

Find the initial velocity of a car which stopped in 10 sec by applying brakes. Retardation due to brakes is 2.5 m/sec sq.

Answer 1

$v = u - at \\ u = v + at$
but, here,
$v = 0 \\ a = 2.5 \\ t = 10$
so,
$u = 2.5 \times 10 \\ u = 25 \: m {s}^{ - 1}$

Answer 2

$25ms { }^{ - 1}$

Explanation:

From the velocity-time relation we know that

$v = u + at$

So,

$u = v - at$

Here,

Initial velocity(u)=u

Final velocity(v)=0m/s

Acceleration(a)=(-2.5)m/s²

Time(t)=10sec

Therefore,

u=(0)-(-2.5*10) (Here (*) is the sign of multiplication)

u=(0)-(-25)

u=25m/s (B'coz( - - )=(+))

Hence, u=25m/s.