Question

The velocity of a particle at an instant is $15m{s}^{-1}$. After $5s$, its velocity will become $25m{s}^{-1}$. The velocity at $4s$, before the given instant will be

• A. $23m{s}^{-1}$
• B. $7m{s}^{-1}$
• C. $25m{s}^{-1}$
• D. $15m{s}^{-1}$

Answer 1

Answer: (B)

Given, $u=15m{s}^{-1},t=5s$ and $v=25m{s}^{-1}$
As, $v=u+at$ .... (i)
where, $v$ is final velocity, $u$ is initial velocity, $a$ is acceleration and $t$ is time.
From Eq. (i) $a=\frac{v-u}{t}$
Substituting given values of $v,u$ and $t$, we get
$a=\frac{25-15}{5}=\frac{10}{5}=2m{s}^{-2}$
Now, velocity at $4s$, before the given instant is given as
$v=u+at$
where, $v=15m{s}^{-1},a=2m{s}^{-2}$ and $t=4s$.
$\Rightarrow 15=u+(2)(4)$
$\Rightarrow u=7m{s}^{-1}$