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In taking this to extended bodies and moving to the first law there
are a number of developments. In place of a particle we deal with a
system, which must be carefully defined; the concept of work (W) will
cover only the mechanical ways of changing the total energy of the
system; non-mechanical ways are called heat (Q);
the total energy of
the system (E ) or (U) will comprise not only the kinetic and various
potential energies of the body but other energies which can be labeled
thermal, chemical, etc. The law is written as W + Q = delta U.
with the first law, which then tells you that the system has lost some
kinetic energy and there has been a heat transfer to the surroundings
(which, in the fullness of time, happens).