Energy , as we'll discuss in this article, represents the total amount of energy in a system. As objects move, the energy associated with them - such as kinetic energy - may convert, but if energy is conserved, then their sum should remain the same.
The law of conservation of energy only applies to systems that are not affected by external forces . A ball rolling on a rough surface does not follow the law of conservation of energy because it is interacting with the surface. In fact, the surface is doing work on the ball through friction. However, if we consider the ball and the surface together, then the law of conservation of energy applies to them. We usually call this combination a ball-plane system .
