This post got me to noodling around online for similar type things. One thing I noticed in the attached conversation is that many, many people seem to have the idea that anything that moves "forever" represents the forbidden perpetual motion machine. They'll note how it "must be possible, since the planets keep going forever." Very few of them seem to get that what's forbidden is perpetual energy output with no source, not simply "motion."
Yes there is a difference between perpetual motion, which is required by Newton's First Law, and a perpetual motion machine. +1 (PMM)
Also there are types of perpetual motion machine.
Your description refers to PMM of the first kind which is forbidden by the First Law of Thermodynamics or conservation of energy as you say.
There is another, trickier, type of PMM called a PMM of the second kind, which is forbidden by
yup you guessed it the Second Law of Thermodynamics.
Probably the most relevant enunciation here is that due to Clausius.
It is impossible to transfer heat from a colder body to a warmer one without net changes in other bodies.
This forbids what amounts to a double first law machine being used as a PMM (that's the tricky bit)
Edited by studiot, 18 May 2017 - 10:49 PM.