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Mechanical energy is the energy which is possessed by an object due to its motion or its stored energy of position. Mechanical energy can be either kinetic energy (energy of motion) or potential energy (stored energy of position).
Objects have mechanical energy if they are in motion and/or if they are at some position relative to a zero potential energy position. A moving wheelchair possesses mechanical energy due to its motion (kinetic energy).
A moving volleyball possesses mechanical energy due to both its high speed (kinetic energy) and its vertical position above the ground (gravitational potential energy). A CD player at rest on the top shelf of a locker possesses mechanical energy due to its vertical position above the ground (gravitational potential energy).
A barbell lifted high above a weightlifter's head possesses mechanical energy due to its vertical position above the ground (gravitational potential energy). A drawn bow possesses mechanical energy due to its stretched position (elastic potential energy).
An object which possesses mechanical energy is able to do work. In fact, mechanical energy is often defined as the ability to do work. Any object which possesses mechanical energy - whether it be in the form of potential energy or kinetic energy - is able to do work. That is, its mechanical energy enables that object to apply a force to another object in order to cause it to be displaced.
Numerous examples can be given of how an object with mechanical energy can harness that energy in order to apply a force to cause another object to be displaced. A classic example involves the heavy ram of a pile driver. A pile driver consists of a massive object which is elevated to a high position and allowed to fall upon another object (called the pile) in order to drive it downwards. Upon hitting the pile, the ram applies a force to it in order to cause it to be displaced.
In our example we are assuming an equal external force pushing each wheelchair. So while there is mechanical energy in the wheelchair, in each example, the external force is stable.
Another example which illustrates how mechanical energy is the ability of an object to do work can be seen any evening at your local bowling alley. The mechanical energy of a bowling ball gives the ball the ability to apply a force to a bowling pin in order to cause it to be displaced. Because the massive ball has mechanical energy (in the form of kinetic energy), it is able to do work on the pin.
The mechanical energy of an object can be the result of its motion (i.e., kinetic energy) and/or the result of its stored energy of position (i.e., potential energy).
The total amount of mechanical energy is the sum of the potential energy and the kinetic energy. This sum is referred to as the total mechanical energy (abbreviated TME).
There are two forms of potential energy discussed here - gravitational potential energy and elastic potential energy. Given this fact, the above equation can be rewritten:
There are conditions under which the total mechanical energy will be a constant value and conditions under which it is a changing value. When work is done upon an object by an external force, the total mechanical energy (KE + PE) of that object is changed. If the work is "positive work", then the object will gain energy. If the work is "negative work", then the object will lose energy.
The gain or loss in energy can be in the form of potential energy, kinetic energy, or both. Under such circumstances, the work which is done will be equal to the change in mechanical energy of the object. When work is done upon an object by an internal force (for example, gravitational and spring forces), the total mechanical energy (KE + PE) of that object remains constant. For example, as an object is "forced" from a high elevation to a lower elevation by gravity, some of the potential energy of that object is transformed into kinetic energy. Yet, the sum of the kinetic and potential energies remain constant.