Why can we use perpetual motion as a source of infinite energy?
We cannot make a perpetual motion machine in practice or in theory. Contrary to one answer, there is no theory in physics or other sciences that hypothesizes a machine with 100% efficiency. In fact, to the contrary, the 2nd Law of Thermodynamics for closed systems states that all useful energy H will eventually convert into useless energy S. It’s the useful energy that systems use (duh) to make that motion. Machines cannot use useless energy (again duh) for motion. We specify the system as closed because the common meaning of perpetual motion means the machine runs forever on whatever energy it started with. In other words, energy wise, the system has to be closed to the outside. For example, if we pump additional usable energy H into a machine, that is not a perpetual motion machine is it? This means a perpetual motion machine has to have a fixed total energy TE. So when there is total energy in a closed system machine, we have TE = H + S because energy is either useful H or useless S
