Question

A man of mass $M$ stands at one end of a plank of length $L$ which lies at rest on a frictionless surface. The man walks to other end of the plank. If the mass of the plank is $M/3$, then the distance that the man moves relative to ground is :

• A. $3L/4$
• B. $L/4$
• C. $4L/5$
• D. $L/3$

Answer 1

Answer: (B)

$L/4$

Let distance moved by the planck be x, then wrt ground the man will move by a distance $L-x$
Now the net displacement of the centre of mass will be zero in ground frame
$M(L-x)=M\ast x/3$
$x=3\ast L/4$
$(L-x)=L/4$