 We notice that while preparing or setting up the operational definition of work, we have used a sign convention. Work is an interaction, so there is a direction involved. Either system A does work on system B or system B does work on system A. So we have to decide which direction is positive. Usually we have a habit of saying left to right is the positive direction, bottom to top is the positive direction. But that is by convention. Nothing prevents us from using downward as the positive direction or the left word as the positive direction. So in this case, our sign convention is that work done by a system which leads to the raise of weight is positive. And work done on a system, mind you that this is our convention and this is the convention which is often used by mechanical engineers and physicists. This is not the only possible convention. An exactly opposite scheme is quite often used by chemists and chemical engineers. However, because we are in the physics and mechanical engineering domain and our background in mechanical engineering, we will be using this sign convention. Thank you.