perpetual motion 
noun [mass noun]
1A state in which movement or action is or appears to be continuous and unceasing: the planet is in perpetual motion the perpetual motion of the dance floor...
- So, it is not good sense to interpret the state of an electron in a stationary orbital as perpetual motion.
- He is in perpetual motion.
- Further, Leucippus proposed that atoms remain in perpetual motion.
1.1The motion of a hypothetical machine which, once activated, would run forever unless subject to an external force or to wear: the age-old quest for the secret of perpetual motion [as modifier]: a perpetual motion machine...
- Next thing you know, we'll find out that these firms are working on cold fusion and perpetual motion machines.
- These findings frustrated the believers of a perpetual motion machine, and angered the industrial tycoons who sponsored the whole endeavor.
- Do I think a perpetual motion machine will ever be invented?