A large rock is placed in a gallery. Instructions for picking it up are posted next to it.
When the viewer attempts to pull the rock away from the center of the earth, it looks like nothing is happening – the rock is too heavy to be picked up, so the viewer, the rock and the planet remain immobile in their respective places. However, if you look at these three elements from a point of view of energy flow, quite a lot is happening. In thermodynamics terms, transfer of energy can be seen in heat emitted and work being done, and in fact the viewer is working as hard as he or she can. Work performed by a system is the energy transferred by the system to its surroundings. For example it can be measured as the force on an object that overcomes a resistive force. Here work is done by resisting the force of gravity and changing the potential energy of the rock. The rate at which work is performed is power.
Energy is one of the most fundamental and universal concepts of physical science, but one that is remarkably difficult to define in a way that is meaningful to most people. This perhaps reflects the fact that energy is not a “thing” that exists by itself, but is rather an attribute of matter that can manifest itself in different ways. It can be observed and measured only indirectly through its effects on matter that acquires, loses, or possesses it.