Sum of a system's kinetic energy (KE) and potential energy (PE). Mechanical energy is constant in a system that experiences no dissipative forces such as friction or air resistance. For example, a swinging pendulum that experiences only gravitation has greatest KE and least PE at the lowest point on the path of its swing, where its speed is greatest and its height least. It has least KE and greatest PE at the extremities of its swing, where its speed is zero and its height is greatest. As it moves, energy is continuously passing back and forth between the two forms. Neglecting friction and air resistance, the pendulum's mechanical energy is constant.

Learn more about mechanical energy with a free trial on Britannica.com.

In physics, mechanical energy describes the potential energy and kinetic energy present in the components of a mechanical system.

Related concepts

When a given amount of mechanical energy is transferred (such as when throwing a ball, lifting a box, crushing a soda can, or stirring a beverage) it is said that this amount of 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 make simplifying assumptions to make calculations about how mechanical systems react. 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 particle(s) 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.

References

Notes