Physics Van 3-site Navigational Menu

Physics Van Navigational Menu

Q & A: Gravitational kick vs perpetual machine?

Learn more physics!

Most recent answer: 09/24/2015
I know that perpetual machines are scientifically proven to be impossible, but when a satellite uses gravity of other planets to accelerates, is it considered a perpetual machine?
- Eduardo (age 39)
Sao Paulo, SP, Brazil

A perpetual machine is impossible because it violates the phycial laws, typically energy conservation. You cannot harvest any work output without investing energy even in an ideal system. In a real-world system, you cannot have a perptual motion without energy input due to dissipative forces, even without any work output.

There are stable orbits around spatial bodies, on which motion is allowed at certain speeds. What you do is to exploit the capability of the spacecraft to change its speed at the intersection of those trajectories by firing the engines accordingly. Graviational acceleration does not fall into perpetual machine category, because the kinetic energy is gained at the expense of gravitational potential energy + fuel driven gas expulsion. There is virtually no gas in space, so dissipative forces are negligible, making such travels feasible by loading a reasonable amount of fuel on the spacecraft.


(published on 09/24/2015)

Follow-up on this answer.