Question

A man and a plank of the same mass are moving with a velocity $v$ along positive $x$-axis. At the same time man jumps along negative $x$-axis with a velocity $v$ with respect to ground, then the speed of the plank is:

• A. $v$
• B. $2 v$
• C. $3 v$
• D. none of these

Answer 1

Answer: (C)

By conservation of linear momentum (consider man and plank as system)
$m v+m v=m(-v)+m v^{\prime}$
( $v^{\prime}$ is the final speed of plank)
$v^{\prime}=3 \,v$