Question

Initially a man and a plank of the same mass $m$ are moving on smooth horizontal surface with a velocity $v\text{m/s}$ along positive x-axis. After some time man jumps along negative x-axis with a velocity $v\text{m/s}$ with respect to ground, then the speed of the plank will be

• A. $3v\text{m/s}$
• B. $v\text{m/s}$
• C. $2v\text{m/s}$
• D. $\text{None of these}$

Answer 1

The correct option is A $3v\text{m/s}$

Initial condition

Final condition

As there are no net external forces in x- direction.
$\therefore$Applying PCLM considering man and plank as system
$\Rightarrow p_f=p_i$
$\Rightarrow mv{}^{\prime }+m(-v)=mv+mv$
$\Rightarrow v{}^{\prime }=3v\text{m/s}$