In
physics, 'mechanical energy' describes the
potential energy and
kinetic energy present in the components of a ''mechanical system''.
Related concepts
When a given sum of mechanical energy is transferred (such as when throwing a ball, lifting a box, crushing a can, or stirring a beverage) it is said that this amount
mechanical work has been done. Both mechanical energy and mechanical work are measured in the same units as
energy in general. It is usually said that a component of a system ''has'' a certain amount of "mechanical energy" (i.e. it is a
state function), whereas "mechanical work" describes the amount of mechanical energy a component has ''gained or lost''.
The ''
conservation of mechanical energy'' is a principle which states that under certain conditions, the total mechanical energy of a system is constant. This rule does not hold when mechanical energy is converted to other forms, such as chemical, nuclear, or electromagnetic. However, the principle of general
conservation of energy is so far an unbroken rule of physics - as far as we know, energy cannot be created or destroyed, only changed in form.
Simplifying assumptions
Scientists often make simplifying assumptions to make calculations about how mechanical systems behave. For example, instead of calculating the mechanical energy separately for each of the billions of molecules in a soccer ball, it is easier to treat the entire ball as one object. This means that only two numbers (one for kinetic mechanical energy, and one for potential mechanical energy) are needed for each
dimension (for example, up/down, north/south, east/west) under consideration.
To calculate the energy of a system without any simplifying assumptions would require examining the state of all
elementary particles and considering all four
fundamental interactions. This is usually only done for very small systems, such as those studied in
particle physics.
Distinguished from other types of energy
The classification of energy into different "types" often follows the boundaries of the fields of study in the natural sciences.
★ Chemical energy, the kind of
potential energy stored in
chemical bonds; studied in
chemistry
★
Nuclear energy, energy stored in interactions between the particles in the
atomic nucleus; studied in
nuclear physics
★
Electromagnetic energy, in the form of electric charges, magnetic fields, and
photons; from the study of
electromagnetism
★ Various forms of energy in
quantum mechanics; for example, the
energy levels of
electrons in an atom
In certain cases, it can be unclear what counts as "mechanical" energy. For example, is the energy stored in the structure of a crystal "mechanical" or "chemical"? Scientists generally use these "types" as convenient labels which clearly distinguish between different phenomena. It is not scientifically important to decide what is "mechanical" energy and what is "chemical". In these cases, usually there is a more specific name for the phenomenon in question. For example, in considering two bonded atoms, there are energy components from vibrational motion, from angular motions, from the electrical charge on the nuclei, secondary electromagnetic considerations like the
Van der Waals force, and quantum mechanical contributions concerning the energy state of the electron shells.
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