Now, before we delve into its application, we must first appreciate its boundaries. The usage of Work-Energy Theorem depends very strongly on the system defined. Consider the case of a ball dropping from a height.
If we define the system as solely the ball, then gravity is an external force, so the Work-Energy theorem says that all the work done by gravity goes into changing the kinetic energy of the ball. Note that as gravity is an external force, there is no gravitational potential energy.