Energy is typically defined as something that gives us the capacity to perform work. As individuals, saying that we feel full of energy is an indicator that we can perform a lot of work. Energy can be classified as one of two types: either as potential energy or kinetic energy.

## Potential Energy

Potential energy is defined as being energy at rest, or energy that is stored. Potential energy may be classified into three groups: (1) energy due to position, (2) energy due to distortion of an elastic body, and (3) energy which produces work through chemical action. Examples of the first group are water in an elevated reservoir or an airplane raised off the ground with jacks; a stretched bungee cord on a Piper Tri-Pacer or compressed spring are examples of the second group; and energy in aviation gasoline, food, or storage batteries are examples of the third group.

To calculate the potential energy of an object due to its position, as in height, the following formula is used:

Potential Energy = Weight × Height

A calculation based on this formula will produce an answer that has units of foot-pounds (ft-lb) or inch pounds (in-lb), which are the same units that apply to work. Work, which is covered later in this site, is described as a force being applied over a measured distance, with the force being pounds and the distance being feet or inches. Potential energy and work have a lot in common.

Example: A Boeing 747 weighing 450,000 pounds needs to be raised 4 feet in the air so maintenance can be done on the landing gear. How much potential energy does the airplane possess because of this raised position?

Potential Energy = Weight × Height

PE = 450,000 lb × 4 ft

PE = 1,800,000 ft-lb

As previously mentioned, aviation gasoline possesses potential energy because of its chemical nature. Gasoline has the potential to release heat energy, based on its British thermal unit (BTU) content. One pound of aviation gas contains 18,900 BTU of heat energy, and each BTU is capable of 778 ft-lb of work. So, when we multiply 778 by 18,900, we find that one pound of aviation gas is capable of 14,704,200 ft-lb of work. Imagine the potential energy in the completely serviced fuel tanks of an airplane.

## Kinetic Energy

Kinetic energy is defined as being energy that is in motion. An airplane rolling down the runway or a rotating flywheel on an engine are both examples of kinetic energy. Kinetic energy has the same units as potential energy, namely foot-pounds or inch-pounds. To calculate the kinetic energy for something in motion, the following formula is used:

Kinetic Energy =

^{1}⁄2 Mass × Velocity^{2}To use the formula, we will show the mass as weight divided by gravity and the velocity of the object will be in feet per second. This is necessary to end up with units in foot-pounds.

Example: An Airbus A380 weighing 600,000 lb is moving down the runway on its takeoff roll with a velocity of 200 fps. How many foot-pounds of kinetic energy does the airplane possess? [Figure 1]

Kinetic Energy =

^{1}⁄2 Mass × Velocity^{2}Kinetic Energy =

^{1}⁄2 × 600,000 ÷ 32.2 × 200^{2} KE = 372,670,000 ft-lb

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