Work Done "On" or "By" The System Will someone help solidy this concept of Work 'On' a system and Work 'Done' by a system. If I lift an object from some defined zero potential. Then the work BY the system is -mgh, because gravity 'g' is in the reverse direction as the displacement 'h', but work done ON the system is +mgh because it is the opposite of work BY the system. But what do I do if I don't know the work done by the system, i.e. there is not a given 'g' vector pointing down? How can I tell if the work is plus or minus?