Question

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:

• A. work done by friction on man with respect to ground is negative.
• B. work done by friction on man with respect to ground is positive.
• C. work done by friction on plank with respect to ground is positive.
• D. work done by friction on man with respect to plank is zero

Answer 1

Answer: (A), (C), (D)

The correct options are
A work done by friction on man with respect to ground is negative.
C work done by friction on plank with respect to ground is positive.
D work done by friction on man with respect to plank is zero

Since the friction between the plank and the person is sufficient.

Therefore, there will not be any relative motion between the plank and the person

So, displacement of the man w.r.t. ground=displacement of the plank w.r.t. ground=let say '$x$'

To, resist the relative motion friction will act in the backward direction on the person

Therefore the work done by the friction w.r.t. ground will be=$f\times x\times \cos \pi =-f\times x=negative$

on the plank friction will act in the forward direction.So, the workdone by the friction w.r.t. ground on the plank

=$f\times x\times \cos 0=f\times x=positive$

Since, there is no relative motion between the plank and the person.So, the displacement of the person w.r.t. to plank =$0$

Therefore the work done by the friction on the person w.r.t. plank=$0$