Why can't a perpetual motion machine work?

A genuine perpetual motion machine, one that runs indefinitely without an external energy source, is not possible because it would violate the laws of thermodynamics, according to the [royalsociety.org](https://royalsociety.org/blog/2018/09/perpetual-motion/) article on the subject. In practice, friction and other forces also steadily drain the energy of anything that moves on Earth. [1]

Physics texts divide these devices into two types. Machines of the first kind violate the conservation of energy, which says energy can be neither created nor destroyed. A device that produced more energy than it took in would do this. The [pbs.org](https://www.pbs.org/video/physics-girl-are-perpetual-motion-machines-possible/) Physics Girl episode sums it up as: energy never comes for free, so if a machine seems to produce motion, heat or light, look for the energy source. [2] [3]

Machines of the second kind violate the second law of thermodynamics. According to a [libretexts.org](https://phys.libretexts.org/Bookshelves/University_Physics/Radically_Modern_Introductory_Physics_Text_II_(Raymond)/24%3A_The_Ideal_Gas_and_Heat_Engines/24.03%3A_Perpetual_Motion_Machines) physics text, internal energy can only be tapped for work when there is a hot reservoir and a cold one. If everything is at the same temperature, no conversion to useful work is possible. Even a perfectly designed heat engine cannot turn all its input heat into work, and no perpetual motion machine has been convincingly demonstrated. [2]

Newton's first law says an object keeps moving at constant velocity unless an external unbalanced force acts on it. MIT engineer Dan Frey notes on [engineering.mit.edu](https://engineering.mit.edu/ask-an-engineer/is-it-possible-to-construct-a-perpetual-motion-machine) that such forces are everywhere. A swinging washer on a string stops because of air friction. A playground swing keeps going only if someone keeps pumping their legs. Perpetual motion would need both an initial force and a sustaining one. [4]

The same MIT answer says the Moon is very nearly a perpetual motion machine, yet it drifts about two centimeters farther from Earth each year, so even it is affected by unbalanced forces. Grandfather clocks look perpetual, but a weight or spring supplies the push through an escapement, and eventually the spring needs rewinding. [4]

Unbalanced wheels are a classic design. The Physics Girl episode explains that a wheel with uneven weights swings like a pendulum until its center of gravity settles at the lowest point, and friction on the axle would stop it anyway. The drinking bird toy keeps dipping, but it draws on the ambient heat of the room: evaporation cools its head, which creates a pressure difference that tips it over. [3]

A physics-education paper in the American Association of Physics Teachers literature, available via [doi.org](https://doi.org/10.1119/5.0102604), says that close analysis of purported machines invariably finds either an assumption that violates thermodynamics or an outside energy input, which means the system is not isolated. [5]