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A question about inertia

  1. Jun 5, 2015 #1
    Force is something that causes acceleration...

    Inertia is something that opposes acceleration...

    Can inertia be understood as some kind of negative force prevelant everywhere in the universe?

    (Hope this doesn't get locked/deleted, these days one can't even think safely)
    Last edited: Jun 5, 2015
  2. jcsd
  3. Jun 6, 2015 #2


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    I don't believe so, no. Especially since a force is defined as something which causes acceleration, not something that resists it.
  4. Jun 6, 2015 #3
    I don't think so. Here's a thought experiment to demonstrate why i do.
    Imagine a mass of 1 kg in empty space at rest wrt you. If you push it, it gains some velocity wrt you. Now, as soon as you stop applying force, it stops accelerating. This is, in essence, inertia. If inertia were thought as a negative force, it would have to act instantaneously. This means an infinite amount of energy has to be spent.
  5. Jun 6, 2015 #4


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    Inertia can be understood as a force, but perhaps not in the mystical sense that you seem to envision.

    If an object is accelerating under a force, one can choose to consider it from an accelerating coordinate system in which the object is always at rest. In this coordinate system the object is subject to two forces. One is the real force which is working in the direction of the acceleration. The other is an inertial force that is working in the direction opposite to the acceleration. Terms like "fictitious force" or "pseudo-force" may sometimes be used to describe the inertial force.

    In the accelerated coordinate system, Newton's second law (f=ma) holds and is consistent with the observed motion of the object -- it is motionless and the net force on it (the vector sum of the real force and the inertial force) is zero. However, Newton's third law (for every force applied by one object on another there is an equal and opposite force from the other on the one) does not apply to the inertial force. It has no third law partner.
  6. Jun 6, 2015 #5


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    Negative forces also cause acceleration, just in the opposite direction as positive forces.
  7. Jun 6, 2015 #6

    Vanadium 50

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    No, because as soon as you try and explain things quantitatively, you will run into the problem that the units don't match. And then you run back into exact;ly the issue that caused so much grief in the E=mc2 thread. And Russ' advice in that thread is still good:

    Don't be that way. You have been disrespectful of the forum, the forum rules, and the forum membership - who I might point out have dedicated many hours of their valuable time to help you - since you showed up. You cannot act this way and expect not to be chastised. Or worse.
  8. Jun 6, 2015 #7
    Beware...the aggressive one comes charging...:))

    As regards E=mc2 thread, I have already told that I was doing it as a layman would do...I am deeply thankful to the staff here for their hardwork...

    BTW you have completely explained what I asked in the post..Thanks.
  9. Jun 6, 2015 #8
    So, has inertia something to do with the nature of space itself??
  10. Jun 6, 2015 #9


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    If you are trying to figure out why a hunk of lead has more inertia than an equal size hunk of wood, contemplating the nature of space is not going to lead you anywhere helpful.
  11. Jun 6, 2015 #10
    Matter is 99.999999 ( I am not sure about the number of 9s) emplty space only...so may be something within space is doing the trick...
    And...if we go upto fundamental particles...they are just point particles (don't have volume even), so may be...

    Just a thought...
  12. Jun 6, 2015 #11


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    How much inertia does empty space have again?
  13. Jun 6, 2015 #12
    Well....I am in no position to argue...but can provide a random thought...

    An infinity of Particles pop in and out of existence in empty space....may be this contributes to the inertia....
  14. Jun 6, 2015 #13


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    Popping out random non-quantitative speculations of what inertia might be is not an appropriate way to do physics and is also against the rules on these forums.
  15. Jun 6, 2015 #14


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    We can close this thread now - it's run to the point of diminishing returns and jbriggs44's last post is quite appropriate.
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