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Energy is the capacity of a physical system to perform work. Energy exists in several forms such as heat, kinetic or mechanical energy, light, potential energy, electrical, or other forms.According to the law of conservation of energy, the total energy of a system remains constant, though energy may transform into another form.
The SI unit of energy is the joule (J) or newton-meter (N * m). The joule is also the SI unit of work
Energy is defined as the ability of one physical system to produce a physical change on another physical system. This energy can take on lots of different forms as it produces this physical change. For example, if I pick a textbook up off my class room floor and place it on the desktop, I have used energy to make this physical change. This also enters into the scientific definition of work, which is a force applied to an object, which results in the object moving some distance. It would be fair to say I did work when I picked the textbook up off the floor and placed it on the desk, because I applied force to the book and moved it a certain distance. This contrasts with holding the book motionless in my hands for 10 minutes. I am still expending energy, holding the book still, because it would accelerate to the floor if I did not exert a certain amount of force on it. But by the scientific definition, I did no work because the book moved no distance.
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