# A MAN OF MASS M STANDS AT ONE END OF A PLANK OF LENGTH L WHICH LIES AT REST ON A FRICTIONLESS HORIZONTAL SURFACE. THE MAN WALKS TO THE OTHER END OF THE PLANK. IF THE MASS OF THE PLANK IS (M/3), THE DISTANCE THAT THE MAN MOVES RELATIVE TO THE GROUND WILL BE _________ A) L. B) L/4. C) 3L/4. D) L/3

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by vatsalyabaranwal

2014-10-18T12:54:39+05:30

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Mass of man = M
mass of plank = M/3
let man is standing at the starting of plank(end A), at x=0
end of plank(end B) is at x=L
CM of man is at x=0
CM of plank is at x=L/2

position of CM of system =

Let after walking to end A from end B, he travels a distance z.
CM of man is at x=z
CM of plank will shift (L-z) towards left,
CM of plank is at = [L/2 - (L-z)] = (z-L/2)
Since there is no external force, CM of system won't change its position.