I have seen this example in several books.
The work done by a porter in carrying a luggage on head while walking on a horizontal force is $0$ as the gravitational force is at $90^0$ with the horizontal plain.
Now,I cannot accept it as I think that the gravitational is not causing the displacement.So,we cannot relate this force with another displacement caused by a different force (here,the muscular force of the porter).
So,am I correct?
Thanks for any answer!!