Waht is the work done by friction when a man is walking? zero or nonzero? [ friction is not causing the displacement of point of contact.....or is it?] I was analysing the work done by friction and i seem to get conflicting answers. If we consider the displacement of point of contact, it is zero, hence work done by friction must be zero. However if we use Work-Energy theorem, as the man acquires some velocity, so some amount of positive work is done. As the only EXTERNAL force which can do work is friction, what is the work done by friction? (zero of positive)?