Energy is the unknown, famous, tool, and goal in our live. The definition of energy is controversial till this time. For example, according to Britannica.com (1) “Energy, in physics, the capacity for doing work. It may exist in potential, kinetic, thermal, electrical, chemical, nuclear, or other various forms. There are, moreover, heat and work”. This is not the only source to define energy in that manner, but many others have taken the same approach. So, the definition’s core is the capacity to do work, but work is actually energy. Given this fact the definition will read energy is the capacity for doing energy which makes the definition adding more ambiguity than clarification. The illusive nature of energy is one of its strongest aspect. To my opinion, the problem in defining energy is unreasonable effort to eliminate time form the context. Time the factor we will never be able to avoid as long as the galaxy keep moving.
One of the units to measure energy is Joule, and according to Dennis Silverman (2) Joule (J) is the MKS unit of energy, equal to the force of one Newton acting through one meter. Another unit is BTU(British Thermal Unit), and according to Dennis Silverman is the amount of heat necessary to raise one pound of water by 1 degree Fahrenheit (F). It is also worth mentioning that.
1 British Thermal Unit (BTU) = 1055 J (The Mechanical Equivalent of Heat Relation)
On the other hand, the source of what we call energy is obviously well known because it raises everyday before our eyes. It shines with its golden beams of light to keep everything moving winds, rains, oceans, rivers, trees, animals, and human. Everyone wants to catch the day to live as much as possible and to absorb as much possible. It is the daily race which has been going on since the life started on this planet.
The strange thing about energy is that; as the energy flow to earth, particles rush to get together to form a big tree or strong muscle or family, and it keeps on feeding for certain time, and then it is the time to reverse the cycle. The particles will go back to the ground and the energy will be released again to start a new cycle with a new generation.
@ Taha Abdelghafar