Is it not inertia that makes a flywheel resist being put into motion? And if that is true, is inertia a consequence of being unable to create energy, i.e., why perpetual motion machines do not in fact work?

Clarification: I found the concept that a heavy object (heavy because it is on Earth but importantly weightless in outer space) would without gravity still resist motion -- as a youngster, I thought weightless would mean that you could push a huge object easily. But of course now I know that a massive object resists being moved in proportion to its mass. Perhaps others were puzzled in the same way at one point.

I also read of Mach's principle which, as I understand it, attributes this resistance to the effect of distant massive bodies -- the gravitational pull of objects even light years away.

What I am simply asking is, why this apparently almost mystical explanation? Would not a simpler explanation be, that if we push a massive object it now has momentum -- if it was a heavy flywheel and you got it moving, you could then run a generator. So since you can't create energy from nothing, it requires energy to get the flywheel to move and this is what we perceive as the resistance called inertia.


closed as unclear what you're asking by John Rennie, stafusa, Jon Custer, Aaron Stevens, knzhou Oct 4 '18 at 13:45

Please clarify your specific problem or add additional details to highlight exactly what you need. As it's currently written, it’s hard to tell exactly what you're asking. See the How to Ask page for help clarifying this question. If this question can be reworded to fit the rules in the help center, please edit the question.


Actually, inertia ("an object in motion remains in motion in a straight line absent an outside force") sustains a kind of perpetual motion. But it's the boring kind, where no energy is extracted from the moving system. It is pretty challenging to imagine mechanics without inertia.

So-called "perpetual motion machines," such as a flywheel that starts moving on its own and can be used to drive some motor, are generally a combination of mechanisms that are allowed under Newton's laws of motion. The trouble arises when you consider the motion of the entire system and its surroundings. Then you run up against the second law of thermodynamics: a statistical observation, which says that you cannot extract useful energy from a system without also moving a larger amount of energy from a high-temperature region to a low-temperature region.

  • $\begingroup$ If one attempts to push a flywheel, what do you call the resistance one perceives? (Other than friction.) $\endgroup$ – releseabe Oct 3 '18 at 11:27
  • $\begingroup$ Rotational inertia is not really different from linear inertia (though a proper explanation is too long for a comment). The important part of this answer is the second paragraph, not the first. $\endgroup$ – rob Oct 3 '18 at 11:34
  • $\begingroup$ I guess perpetual motion is not the central idea: i mean, if not for inertia, i could push a flywheel to high rpms without expending energy but then use to flywheel to, say, make electricity. And in this way, energy would be created. $\endgroup$ – releseabe Oct 3 '18 at 11:46
  • 6
    $\begingroup$ @releseabe The problem is that to get energy out of the flywheel, you are taking it from the inertia of the wheel. The reason you can get the energy out of a flywheel is the same reason it takes energy to get it moving. $\endgroup$ – JMac Oct 3 '18 at 11:48

Another way to think of inertia, at least in Newtonian mechanics, is that inertia is the property of matter that relates kinetic energy to velocity through the equation $KE = mV^2/2$, where m is the mass of the moving object. For there to be no inertia would mean that m is zero for all matter.

In the case of a flywheel, if you do all the math you find the same thing: for the rotational inertia of a flywheel of any finite extent to be zero, its mass must also be zero. So, regardless of its rotational velocity, it would carry no kinetic energy.

When relativity and quantum mechanics are taken into account, a massless object can only move at the speed of light. A photon is massless, moves at the speed of light, and carries energy - but not exactly "kinetic energy" because its energy is independent of its velocity and depends instead on its wavelength.


Not the answer you're looking for? Browse other questions tagged or ask your own question.