Mechanical energy is the sum of energy in a mechanical system. This energy includes both kinetic energy, the energy of motion, and potential energy, the stored energy of position. A mechanical system is any group of objects that interact based on basic mechanical principles. Typically, in a mechanical system, gravity is the only major outside force that needs to be considered. In a chemical system, on the contrary, the forces between individual molecules and atoms must all be considered.

Mechanical energy exists as both kinetic and potential energy in a system. Kinetic energy, the energy of motion, exists whenever an object is in motion. Potential energy is based on the position of an object. It can not cause any change on its own, but it can be converted to other forms of energy. A bowling ball suspended ten feet above the ground, for example, would possess no kinetic energy. It would, however, possess a large amount of potential energy that would be converted to kinetic energy if the ball were allowed to fall.

Many assumptions are made to simplify calculations concerning mechanical energy and mechanical systems. Objects in such systems, for example, are treated as individual objects instead of as sums of billions of molecules. Calculating both the kinetic and potential energy of a single object is a simple task; calculating kinetic and potential energy for billions of tiny molecules would be nearly impossible. Without making such simplifying assumptions when dealing with mechanical systems, scientists would need to examine the individual atoms and all of the interactions and forces existing between them. Such tasks are typically reserved for particle physics.

High school physics classes usually begin by instructing students about the basic principles of mechanical systems and mechanical energy. This is because they are easy to visualize and easy to simplify. Basic calculations concerning mechanical systems can be made without even using calculus. In most basic physics problems, the mechanical system remains closed and factors that would normally remove energy from the system, such as friction and air resistance, are ignored.

There are many other forms of energy, and sometimes it can be difficult to properly distinguish one from the other. Mechanical energy strictly concerns the kinetic and potential energy present within mechanical systems. Chemical energy focuses on the energy stored within chemical bonds in molecules. Nuclear energy is concerned with the energy present in interactions between particles in an atom's nucleus. Mechanical energy generally ignores the composition of objects and focuses only on the objects in question, without concern for their molecular makeup.