Energy is an unusual concept in physics. Physics defines energy as the ability to do Work. Hyperphysics explains the concept of Energy as a “currency” of work:
Energy is the capacity to do work; You must have energy to do work, it is like the currency for performing work. To do 100J of work, you must expend 100J of energy.
Energy can be in many types – there is the energy of movement (Kinetic Energy), and energy of sound (Sound Energy), Light Energy, Heat Energy, Electric Energy and so on. There is another type of Energy, Potential Energy. This is “Energy waiting to happen”, or stored energy. Some examples of this are Gravitational Potential Energy and Elastic Potential Energy.
What do we mean by Potential Energy (Stored Energy or “Energy waiting to Happen”)? Well the best answer to that involves a concept called Conservation of Energy, which states:
Energy in a closed system can neither be created nor destroyed, it merely changes forms.
This is actually a complicated concept. It does not mean that objects cannot lose a type of energy, i.e. objects cannot lose kinetic energy, but that if they do lose one type of energy they must gain another type of energy. For example, an object shot upwards gradually slows down (thus losing kinetic energy), but as it does, it gains gravitational potential energy – it has energy ‘stored’ by its position above the surface. If it were to be stopped at the maximum height (for example by sticking to a ceiling), it could later fall, and thus gain kinetic energy again. The kinetic energy it had was “stored” until it began to fall again.