As we have already discussed, the work done on a system is equal to the change in energy of the system caused by a force external to the system. In addition, in order for an external force to do work on a system, the point of application of the force must move. In this class, when we discuss work done on a system, we are typically discussing motion of the entire system, not just the point of application of the external force. Therefore, we typically say that, in order for an external force to do work on a system, the system must undergo a displacement while the external force is being applied.
If an external force applied to a system and is pointed in the same direction as the displacement of the system, the work done on the system by the external force is positive .