How to understand the work-energy theorem?
I took a short lecture on physics for engineering last week. The lecturer emphasized that the work done on an object will cause the kinetic energy change as
$$W = \Delta \text{KE}.$$
I know this concept might be so common to you but to me, as a beginner, it is pretty hard to understand the reason. My understanding is that 'work' is the energy an external object 'injects into' the object or is the energy an external object 'takes away' from the object. I think the work done by the object should equal to the total energy changed on that object, which could be in any form (heat, potential or kinetic energy.) Why does the theorem only explicitly refer to kinetic energy? Will this theorem work in some cases or in all cases?