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How much force will get this thing moving?

  1. Aug 26, 2006 #1
    if there is an object with zero inertia, and there are very strong forces pulling at the thing all around, but the forces all cancel each other out (so the object stays staionary) will it take any effort to start it moving?

    i find myself being able to argue both ways.. help
  2. jcsd
  3. Aug 26, 2006 #2


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    If I'm understanding your question correctly, then the answer is the same as if the object did have inertia. Any imbalance in the forces, or addition of another one, will cause a displacement.
  4. Aug 26, 2006 #3

    It would not be the same. In Forgot's scenario, the force to move his object is 0, which is why the conundrum exists. If the object had mass, however, we can plug in numbers to get a real force.

    [tex]\mbox{no inertia}\;F=0\quad \mbox{has inertia}\;F>0[/tex]

    The two forces are therefore not equal.


    Though your question is unanswerable, the important thing, and perhaps what Danger meant to say, is that all of the forces, being in balance, equal a net force of 0. Which means it would be the same situation if no forces where applied at all.

    I would suggest the only possible way to conceptually answer this question is to state that since the F=ma equation is unresolvable, the only possible explanation is that the object is travelling at C.

    Your object is a photon!
    Last edited: Aug 26, 2006
  5. Aug 26, 2006 #4


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    I see what you mean, Why, but I'm confused. Forgot specified that the object remains stationary, so it has to have mass. I have no idea how he expects something with mass to have no inertia, but that's beside the point for now. Something without mass, as you said, must travel at light-speed (as far as I know). So even if the inertia is zero, wouldn't some input be required to move mass? :confused:
  6. Aug 26, 2006 #5
    A rehash: If we imagine the object has very small inertia but is stationary because the forces acting upon it are in balance, it won't take much effort to get it moving. If it had even less inertia it would take even less effort. You can carry on with this, but if you go all the way to zero you have to ask what those forces acting upon it have got to bite on. It's a bit like dividing by zero, things don't make sense any more.
  7. Aug 26, 2006 #6

    That is an interesting observation.

    Are the following considered to be true:

    All matter has mass.
    All mass has inertia.
    To remove inertia, hence mass, hence matter, means to convert it to energy.
    Energy has no inertia.​

    Curious to hear the answers to those...
  8. Aug 26, 2006 #7
    I'm not too sure about answering that, WhyIsItSo. Maybe matter, mass, and inertia are all different ways of looking at the same thing. A thing with a resistance, whatever that thing might be. But I am pretty sure that energy is something that things have, and isn't something that can exist in its own right. Rather like the colour red: something can be red, but you can't have red on its own. And "energy" maybe stops people from thinking about what things are.
  9. Aug 28, 2006 #8
    if an object has no inertia, it means that there would be no reluctance to motion, which means that object would need to be massless, and travelling at the speed of light, c.

    at c, the photons are not accelerating, and hence there is no net force. if you apply a force, they can't accelerate either. neither will they slow down, as there is only one speed that they can go.

    thus, the object would not be stationary as in your question.
  10. Aug 28, 2006 #9
    energy has no inertia? photons have momentum, which can be loosely related to inertia, so that solves the problem of the conservation of momentum.
    (just making a wild guess)
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