Energy
Energy is defined as the ability of a system or body to work.
Energy conservation law: "energy is neither created nor destroyed, it is only transformed"
When we tallk about sustainability we refer to the methods "for meeting current generation requirements, without compromissing the future of our planet.
Main sources of energy
-Renewable: do not get exhausted when they are used for energy. They regenerate at the same rate that they are used or even faster. Sun (hydroelectric, tidal, wave (solar), wind eolic) earth (geothermal), water
-Non-renewable: They get depleted as they are used, either because they don´t regenerate or because they regenerate more slowly than they are used. (Fossil fuels, Nuclear eneregy)