Why are stable orbits not considered to be perpetual motion machines?


If a satellite is brought into a stable orbit around another body, it should keep orbiting, right?

In: Physics

It’s not perpetual. A “stable” orbit will still lose some energy and the satellite will eventually be flung off into space or come crashing down. For example, the moon seems stable and over human terms it is, but it’s still getting further away by about an inch a year.

A perpetual motion machine means that it’s creating usable energy from nothing. This violates basic physics. The energy has to come from *somewhere*.

Sure. If you are asking about perfect “high school physics” examples.

In reality no such drag-less 2-body system exists.

Energy is lost to tidal effects, drag and collisions with small objects. Orbits are distorted by other bodies exerting influence on the 2-body system.

If you try to extract energy from this “perpetual motion machine”, you will further degrade its orbit.

It’s just a rotating system with a huge store of kinetic energy, like a big flywheel. The common definition of a perpetual motion machine is one that energy can be extracted from without using up an energy reservoir, however large, that eventually runs out. Even a body as large as a planet loses energy eventually. Collisions with occasional dust and gas, tidal forces that squeeze and stretch the body such as those that have already slowed the rotation of the moon so one side faces us and is infinitesimally doing the same to the Earth.

Well, first, there’s no such thing as a perfectly stable orbit. *All* orbits decay eventually, even if it takes billions or trillions of years.

The second is that a 2 body system isn’t doing any work. A perpetual motion machine is one that can *do work* indefinitely without an energy source or without depleting an energy source, but a planet orbiting a star isn’t doing any work, If you tried to extract that energy to do work, you’d be reducing the specific orbital energy of the system. Reduce that energy enough and your orbit decays and your bodies collide. There’s no free lunch (or energy) in physics.

Okay, so I now understand that 2-body systems do no work so we can’t extract any usable work from them without affecting the orbit. Flywheel analogy really helped.

The follow up question I have is would two point masses orbiting around a common center of mass have their orbits decay (this is empty space with no other objects affecting the orbits). This is because most replies state that effects like tidal forces and drag from other particles will cause orbits to decay.