So I would like to know the directions of friction acting on the man and the plank in the following situation:
The man walks on a rough plank (upper surface rough) kept on a smooth floor.
My idea was that there is no external force and hence friction acts in forward direction on the man (to make him move forward) and backward on the plank (as it moves back)
The actual question asked to find the signs of work done by the friction.
Seems like I was right about the friction on plank direction but then they gave the answer that work done by friction on man is negative which conveyed the message that friction acted backward on the man! Wouldn't this violate Newton's third law since both frictions act backward?