What actually is energy? Is it a property associated with matter or just a number? By doing work are we changing the energy of the body or converting one form of energy into other which is already present.
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I suppose that one way to look at energy is that it is a convenient tool for book-keeping tool since it is a conserved quantity. It's actually amazing that there is something that we call "energy" which can exist in so many different forms (e.g., kinetic energy, gravitational potential energy, electrostatic energy, electromagnetic energy, etc.) but that if we sum up all of these individual energies then the total sum is always constant no matter what happens.
As for "work", that is typically defined as the energy associated with exerting a force 'F' on an object for a certain distance 'd'. So it's the energy Fd that is lost by whatever person, being, or contraption does the work and goes somewhere else. The energy can go directly into the object that is being subjected to the force F (say, by letting the energy Fd get transferred into increasing the kinetic energy of the object), but that's not necessarily the case. If the object is being pushed a rough surface, then all that Fd energy may instead get entirely transferred into heat energy associated with the friction.
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What actually is energy?
Energy is a physical quantity that happens to have interesting properties. For example, it was demonstrated, using the laws of Newton which were not formulated having in mind the concept of energy, that for a free falling body $mgh + mv^2/2=ct$, a relation that simplify various calculations. The first term was named potential energy the second kinetic energy. This is the birth of the concept of energy.
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