I know that the net work = the change in kinetic energy, delta KE. But what if the object has both the potential energy and kinetic energy, for example, a falling object, then how can I find out the work done on the object at a particular point? Still the change in KE ignore PE? Also, I know the external work = the change in potential enery, delta PE. It happens in a lifting a brick, but Is that the net work? What's the difference? In addition, work done by gravity= -delta PE. If I am asked to find out the net work, should I plus together the external work and work done by gravity? In this lifting case, is there a kinetic energy? Can I find out the net work by using change in KE? I am confused with some concepts, hope you can help. thanks.