The law of conservation of energy is that energy cannot be created or destroyed, but it can be transferred or transformed from one form to another (including transformation into or from mass, as matter). The total amount of energy in a closed system never changes.
Energy in a system may be transformed so that it resides in a different state. Energy in many states may be used to do many varieties of physical work. Energy may be used in natural processes or machines, or else to provide some service to society. The generic name for a device which converts energy from one form to another is a transducer. Energy transformations in the universe over time are generally characterized by the various kinds of energy available since the Big Bang, including gravitational energy and electromagnetic waves.
Example: Suppose you push a book across a table. The book has kinetic energy but its motion decreases as it slides. It seems like the book's energy is being lost. Actually, the kinetic energy is not lost, it just changes form. The kinetic energy is converted by friction into thermal energy, another form of energy. The thermal energy makes the book, the table, and the air a little warmer.
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Energy cannot be created or destroyed - it can only be converted from one form to another.