Conservative and Non-conservative Forces in Physics

The whole distinction between these two kinds of force comes down to one question. Does the work depend only on the endpoints, or does the route itself cost something? Gravity and a spring do not care how you got there, and that indifference is exactly what makes a potential energy possible. Friction does care, because it acts along the path the whole way, and that is why it can never be stored as potential energy and always ends up as internal energy instead. The blocks give the test for each kind, the work each one does, and the accounting rule that links the nonconservative work to the change in mechanical energy.