A man is standing on a plank which is placed on smooth horizontal surface. There is sufficient friction between feet of man and plank. Now man starts running over plank, correct statement is/are:

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Work done by friction on man with respect to plank is zero.

Work done by friction on man with respect to ground is positive

Work done by friction on plank with respect to ground is positive.

Work done by friction on man with respect to ground is negative.

answer is A, C, D.

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Detailed Solution

At contact point of man and plank, the relative motion is zero. As plank moves backward, man's foot also moves backwards, but friction on man acts forward. Hence work done by friction on man is negative. But with respect to plank, foot will be at rest, hence no work done by friction on man w.r.t. plank.