According to the ideas of modern physics, energy is the substance from which all things in the Universe are made up. Energy can take many different forms: e.g., potential energy, kinetic energy, electrical energy, thermal energy, chemical energy, nuclear energy, etc. In fact, everything that we observe in the world around us represents one of the multitudinous manifestations of energy. Now, there exist processes in the Universe which transform energy from one form into another: e.g., mechanical processes (which are the focus of this course), thermal processes, electrical processes, nuclear processes, etc. However, all of these processes leave the total amount of energy in the Universe invariant. In other words, whenever, and however, energy is transformed from one form into another, it is always conserved. For a closed system (i.e., a system which does not exchange energy with the rest of the Universe), the above law of universal energy conservation implies that the total energy of the system in question must remain constant in time.