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Why is mass dependent on inertia and not vice versa?

  1. Feb 3, 2015 #1
    in here:

    Mass as a Measure of the Amount of Inertia
    All objects resist changes in their state of motion. All objects have this tendency - they have inertia. But do some objects have more of a tendency to resist changes than others? Absolutely yes! The tendency of an object to resist changes in its state of motion varies with mass. Mass is that quantity that is solely dependent upon the inertia of an object. The more inertia that an object has, the more mass that it has.

    they say mass is a measure of inertia... why is that the case?
    why isn't inertia a measure of mass?
    Last edited: Feb 3, 2015
  2. jcsd
  3. Feb 3, 2015 #2


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    Staff: Mentor

    Because there are no units for inertia? I think historically it was just a matter of realizing they are basically the same thing.
  4. Feb 4, 2015 #3
    It is historical. The word, "inertia", is Latin for "idleness", among other meanings. Latin was used by Newton, Kepler, and many others. The word carries on in modern treatises about motion because the original treatises were written in Latin and it's good to know the roots. The word is there to make you question where/how these ideas originated (more Latin).
    An object has "idleness" (inertia, remains at rest) and stays that way unless acted upon by an outside force.
    Does that help? Mind you, I'm putting words into Newton's mouth and I probably shouldn't be. I neither read nor write Latin. I've never read the original works, but given the meaning of the word, "inertia", I can guess where it was used. There wouldn't be a connection to the word, mass. Perhaps there are a few here who might know better than I.

    It's a good thing to know the roots of the words used and where they are/were used. This post was generated due to my own horrible lack of knowledge of Latin. It in no way means to answer, definitively, the original post. I'd be just as curious, but I bet my shot in the not-so-dark hit fairly well close to the mark.

    P.S. Keep an eye out for Latin and one may find it everywhere in the romance and Germanic languages. It is prevalent in chemistry, biology, medicine and other studies,
  5. Aug 4, 2015 #4
    HI there
    i had the same doubt in my mind when i read the same article at same site( i.e. physicsclassroom.com).

    i noticed that i'm not the only one with such doubt and i mailed the creator, Tom Henderson with that question and here is what Tom Henderson has to say in reply

    " Here it is:

    Inertia is the observable quantity. We observe inertia every time we push on an object. The push is resistance by the object. Heavier objects resist the push more than light ones. This tendency to resist a change in a state of motion would exist even if we did not have any words like heavy or light or mass. Inertia is a fundamental property of that can always be observed. On the other hand, mass is a concept we have created to describe observables, one of those observables being the resistance to changes in the state of motion. It is in this sense that mass is a measure of the amount of inertia an object possesses.

    Post if you wish
    . "

    hope this helps. this answer is directly his word and has no chance of being wrong unless Tom Henderson himself is wrong and i doubt of him being wrong.
  6. Aug 6, 2015 #5
    The word "inertia" gives no extra meaning, compared to the word "mass". The statement, "Mass is a measure of the inertia" does not convey any meaning, since then we raise the question, "What is inertia?" Notice that a statement such as "heavier objects resist change in motion better than light ones do" raises the question, what is heavy and what is light? This brings us right back to where we started.

    Modern textbooks on physics emphasize the operational definition involving force, mass and acceleration. I refer you to standard textbooks such as the one by Knight.

    The point is that you don't need the word "inertia" for a logical development of the concepts of physics.
  7. Aug 9, 2015 #6
    hey whats the name of the standard book by Knight? i hope online PDF is available, right? thanks in advance.
  8. Aug 9, 2015 #7
    Physics for Scientists and Engineers, A Strategic Approach with Modern Physics, by Randall D Knight. ISBN: 978-0321740908
  9. Aug 9, 2015 #8


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    Homework Helper

    I would prefer "coexists with" as opposed to "solely dependent on".
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