Huey S. answered • 06/19/19
Exceptional Physics Teacher - AP, IB, High School through University!
The Work-Energy theorem states that we can account for all energy before and after an event, even if it is lost to non-conservative forces. For entry level physics, this 'loss of energy' comes in two forms - Friction, and 'other work'. The beauty of the 'Work Energy theorem' is that we are working entirely with 'Joules', the unit of energy.
I kinematics, there were many items to track, such as displacement, velocity, acceleration, friction, etc. The 'W-E' theorem simplifies that by accounting for the energy in a system, and tracking the energy exchange from one form (potential) to another form (kinetic). Some professors label PE plus KE as 'Mechanical Energy'.
To answer your question specifically, IF PE is not completely converted to KE, that should launch a search into where did the missing energy go? Was it friction - a 'non-conservative' force when multiplied by displacement is a form of energy we do not get back, or some other energy we have not taken into account yet. One thing remains certain - IF your energy before, and energy after are quite different and can not be accounted for in your significant digits, then it's time to start searching for where that missing energy went.
