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In general, the concept energy refers to “the potential for causing a change” The word is used in several different contexts. The scientific use has a precise, well-defined meaning, whilst the many non-scientific uses often do not.
For example, kinetic energy is the amount of work to accelerate body, gravitational potential energy is the amount of work to elevate mass, etc. Because work is frame dependent ( = can only be defined relative to certain initial state or reference state of the system), energy also becomes frame dependent.
For example, a speeding bullet has plenty of kinetic energy in the reference frame of non-moving observer, but it has zero kinetic energy in proper (comoving) reference frame-because it takes zero work to accelerate a bullet from zero speed to zero speed. Of course, the selection of a reference state (or reference frame) is completely arbitrary-and usually is dictated to maximally simplify the problem to be dealt with:
Newton's second law is a mathematical definition of force, first proposed by Newton himself (thus the name).