What is energy? relating to physics

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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...

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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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