Perpetual Motion: Definition & Overview

Instructor: Richard Cardenas

Richard Cardenas has taught Physics for 15 years. He has a Ph.D. in Physics with a focus on Biological Physics.

In this lesson you will learn about the idea of perpetual motion. You will see that perpetual motion is related to the idea of conservation of energy, machine efficiency, and thermodynamics. You will learn whether it is possible to have machines run under the principle of perpetual motion.


Many of us would like to have a machine, like a car, that would keep running without the need for batteries or fuel. This is the main idea behind the concept of perpetual motion. Perpetual motion is defined as any motion that can be sustained without any external sources of energy. It would be like that Energizer bunny in the commercial that keeps going and going forever with only one battery. Of course we know that the Energizer bunny will eventually stop and will only restart when you replace the battery. Giving the bunny a new battery is supplying a new source of energy, therefore the bunny does not obey the concept of perpetual motion (regardless of what the commercial says).

There are two types of perpetual motion: perpetual motion of the first kind, and perpetual motion of the second kind. Perpetual motion of the first kind is defined as any type of motion that once activated, will keep running without any additional source of energy. Perpetual motion of the second kind is defined as motion that gets its energy to run from a source that is at a lower temperature than the temperature of the perpetual motion device. In the next section, we will see if these two types of motion are possible.

The Science and Myth of Perpetual Motion

To understand perpetual motion, we should talk about some of the laws involved in the concept. The first law of thermodynamics is another form of the conservation of energy principle. According to the conservation of energy principle, the total energy of an isolated system remains constant. An isolated system is a system that isn't able to exchange matter and energy with its environment. A simple example would be a thermos bottle. The bottle is supposed to keep its contents hot or cold by isolating the contents from its surroundings. It is fully insulated from any exchange of energy (like heat) or matter. Another form of the conservation of energy principle states that for an isolated system, energy can neither be destroyed nor created, just transformed from one form to another.

According to the concept, once activated, a perpetual machine of the first kind would keep on running without any additional sources of energy. Here is the problem: when any machine runs, there's friction, and friction tends to convert the energy needed to run a machine into heat, slowing the motion down. Therefore, we'd need to supply more energy to the machine to keep it running forever. A perpetual motion machine of the first kind is impossible.

The second law of thermodynamics basically states that the entropy (or disorder) of an isolated system never decreases because systems like this always tend toward something called 'thermodynamic equilibrium', or a state of balance. There were several different versions of this law expressed by theorists, but the important one here is the one made by Clausius. He stated that heat transfer can only occur in one direction, from the hotter body to the colder body.

The perpetual motion machine of the second kind gets its energy to run from a source that is at a lower temperature than the temperature of the perpetual motion device. This is in direct violation of the second law of thermodynamics. This is in obvious violation of the statement that the flow of energy is only from hot to cold. The flow of energy for this second kind of perpetual motion is from cold to hot.

Unfortunately, there are no such things as perpetual motion machines. Every machine man has ever made needs an external source of energy to keep on running. There are some machines out there that approximate perpetual motion like the drinking bird toy shown in the figure below.

The Drinking Bird Toy
Drinking Bird

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