Question

A particle is thrown up with velocity $20\text{m/s}$. Find the time when velocity is $10\text{m/s}$.
[Take $g=10{\text{m/s}}^2$]

• A. $1\text{sec}$
• B. $2\text{sec}$
• C. $3\text{sec}$
• D. $\text{Both (a) and (c)}$

Answer 1

The correct option is D $\text{Both (a) and (c)}$

As per the question, when the particle will be thrown up, at a certain height the particle will have same magnitude of velocity, the direction will be upwards while going up and downwards while coming down.
So, taking the given final velocity as $v=\pm 10\text{m/s}$
initial velocity, $u=20\text{m/s}$
We know from the equation of motion
$v=u+at$
$\Rightarrow \pm 10=20-10t$
From this equation, we get two values for time i.e.
$t_1=\frac{10-20}{-10}=1\text{sec}$
and, $t_2=\frac{-10-20}{-10}=3\text{sec}$