Power. Energy conservation and efficiency

In nature energy is neither lost nor created from nothing; it only changes from one form to another or passes from one body to another. This is the law of conservation of energy: the total energy of a closed system is constant. In real machines part of the energy spent turns into useless heat because of friction and warming. Efficiency is the ratio of useful work to total (spent) work, as a percentage: η = (A_useful/A_total) · 100%. Efficiency is always below 100%, because friction can never be removed completely. For example, when lifting a load up an incline, the useful work is mgh for raising the load, while the spent work is the work done by the pulling force. The larger the efficiency, the better the machine uses energy.