In the late 1600s, Sir Isaac Newton penned a famous law: “Every body remains in a state of constant velocity unless acted upon by an external unbalanced force.” The first part of the sentence (up to “velocity”) suggests that perpetual motion is not only possible but inevitable for any object already in motion. The second part of Newton’s first law of motion, however, throws a wrench in the process. As it turns out, “external unbalanced forces” – non-zero net forces outside applied to the object by another object – are everywhere in our universe.
Dan Frey, an associate professor of Mechanical Engineering and Engineering Systems, explains it this way: “If you take a metal washer and put it on the end of a string and start it swinging, it goes back and forth but eventually it stops. This is because of friction with the air. A playground swing is a different kind of pendulum,” he notes, “but you can keep it going by pumping your legs. If you could pump forever, you would swing forever; but once you remove that energy, you soon stop. Perpetual motion requires an initial force and a sustaining force.”
As it turns out, the moon is very nearly a perpetual motion machine. It goes around the earth every month and has been doing so at almost constant speed for a very long time. Even so, with advanced instruments and careful measurements we can determine that the moon’s motion is changing: it gets farther away from the earth on average by about two centimeters each year. Why? Because even in space there are unbalanced external forces. For objects here on earth, the forces are relatively large and tend to slow motions down after a short period of time. For objects like the moon, the unbalanced forces are small compared to what would be needed to slow down such a large object, so the changes are very slow.
Frey swings back to pendulums. “Grandfather clocks rely on a pendulum that appears perpetual, but in fact it is only engineered to act that way. We use a weight to provide push and gears to modulate the force of the weight. A mechanism called an escapement ensures the push is always in the right direction, counterbalancing the drag force on the pendulum. Carefully designed, a grandfather clock exhibits short-term perpetual motion. But inevitably, its spring needs to be rewound.”
Is perpetual motion possible? According to Frey: No, but things can be engineered to approximate or mimic it. “The laws of physics indicate that perpetual motion would occur if there were no external unbalanced forces,” he says. “But there are. Only by engineering a solution by which an object in motion can consume some store of energy or gather energy from an external source can we approximate perpetual motion.” —Jason M. Rubin
Thanks to Suresh Vishwanathan from Bangalore, India, for this question.