# Motion and Kinetic Energy: a question for science experts **URL:** \ **Category:** Factual Questions\ **Created:** [December 31, 2013, 10:01am UTC](https://boards.straightdope.com/t/motion-and-kinetic-energy-a-question-for-science-experts/677553 "2013-12-31T10:01:38Z")\ **Posts on this page:** 1\ **Showing post:** 9
**Author:** ![Asympotically\_fat](https://avatars.discourse-cdn.com/v4/letter/a/e47c2d/32.png) [@Asympotically\_fat](https://boards.straightdope.com/u/Asympotically_fat)\ **Post date:** [December 31, 2013, 6:35pm UTC](https://boards.straightdope.com/t/motion-and-kinetic-energy-a-question-for-science-experts/677553/9 "2013-12-31T18:35:05Z")
> [@Learjeff](#): > > I thought so, but as it turns out, I was probably wrong. The explanations surpass my understanding, but I believe the predominant interpretation is that if there is only one object in the universe, it by definition isn’t rotating. There will be no (apparent) centrifugal forces at the skin. > > If you fire rockets to make it rotate, then it’s no longer a single object in the universe, and the rotation will be with respect to the exhaust of the retros. If you spin a mass inside, causing the rest to rotate the opposite way, no doubt you’ll get the same kind of effect. > > Hopefully, someone like Chronos or Astymptotically Fat who actually has a clue what they’re talking about will be by to clarify. I deeply resent the idea that I know what I’m talking about! As **leachim** says this is Mach’s principle. It has the status of being an idea that a lot physicists really like, but there aren’t really any theories worth talking about that are truly 100% Machian (i.e. all inertial effects are due to interactions between matter). Despite the fact that Einstein obviously held Mach’s principle in high esteem, general relativity is not Machian. Whether an observer feels inertial forces in relativity is related to geodesics which are defined locally. I.e. you don’t need to look at the whole of space to decide which inertial forces someone feels in GR. --- _[View the full topic](https://boards.straightdope.com/t/motion-and-kinetic-energy-a-question-for-science-experts/677553)._