Energy, in physics, is the ability for a system to do work. Work is the force a system produces on another system over a distance. Therefore, energy is equal to the ability of a system to pull or push against other forces. Mechanical energy is the sum of all of the energy within a system. Mechanical energy can be broken into two forms of energy: kinetic energy and potential energy.

When an object is in motion, the type of energy on display is kinetic energy. Some of the many forms of kinetic energy include rotational (energy from spinning around an axis), vibrational (energy from vibration) and translational (energy from the movement from one location to another). The equation to solve for the amount of kinetic energy of an object at a given time is: KE = (1/2) * m * v^2, where m = the mass of the object and v = the velocity of the object.

Where kinetic energy is the energy of motion, potential energy is the energy stored in an object depending on its position. In this form, the energy is not doing work, but it does have the potential to be converted to other energy forms. In the case of mechanical energy, potential energy transforms into kinetic energy when the object is set into motion. Two forms of potential energy are gravitational and elastic potential energy. Gravitational potential energy is the energy of an object depending on its height above the ground. Elastic potential energy is the energy stored in an object that is stretched or compressed, like a spring.

The law of conservation of energy is a fundamental law of physics and states that within a system isolated from its surroundings, the total energy within the system is conserved. That is to say, though the amount of kinetic energy and potential energy can change from moment to moment the total amount of energy, an object's mechanical energy, never changes as long as it remains isolated. The potential energy of an object is defined by the equation: PE = mgh, where m = the mass of the object, g = the gravitational acceleration and h = the object's height above the ground.

The mechanical energy of a system is the sum of the kinetic and potential energy within the system: mechanical energy = potential energy + kinetic energy. The result of this equation is called the total mechanical energy. Mechanical energy is measured in units called joules. Objects with mechanical energy are either in motion or have stored energy to do work. While an isolated system conserved its mechanical energy, this doesn't typically happen in the real word because some potential energy is transformed into other forms of energy, such as heat, through air resistance and friction. This energy becomes "lost" to the system.