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Inertia and momentum

  1. Jun 20, 2012 #1
    Can we say that momentum is the reason behind inertia? like momentum explains inertia.

    Momentum:- mass * velocity

    net force is the rate of change of momentum.



    Inertia:- it is the tendency of an object to stay in the state of motion it currently is in i.e resist the change of state of motion.

    For example:- a car speeds up (accelerates) but later at some point the velocity becomes constant(assume there is no other force acting on the object).now the net force on the object is zero(so is acceleration) but the object continues to move with constant velocity and hence it has the same momentum (since mass isn't changing and neither is velocity). so no force but momentum is still there.

    momentum is the energy which the moving object has right?
    when there is some velocity the momentum remains constant and the object keeps moving because of this and when the velocity is zero then momentum is zero and thus the object remains at rest.

    here not changing its velocity is the the objects 'tendency' we call it inertia and momentum is what keeps the object moving.

    so what i think is:
    inertia tells us that the state of motion will not change and
    momentum tells us the reason behind it(it is the reason) sort of like "it happens" and "why it happens"

    am i right right? or i am confusing them both?
  2. jcsd
  3. Jun 20, 2012 #2
    Yeah inertia is resistance to motion; momentum can be thought of as a measure of inertia.
  4. Jun 20, 2012 #3
    momentum is a measure of energy-movement

    inertia just says that can't change unless an outside energy hits it
  5. Jun 21, 2012 #4
    So are momentum and inertia connected or not?
  6. Jun 21, 2012 #5
    But isn't mass the measure of inertia?
  7. Jun 21, 2012 #6
    Mass is a measure of inertia, and so is momentum.
    See http://en.wikipedia.org/wiki/Inertia

    For me, I think inertia is a concept invented just to explain the concept of mass and momentum.
  8. Jun 21, 2012 #7


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    I think it's not helpful to think in terms of one being the reason for the other.

    The concept of inertia is subsumed in the concept of momentum.
    What I mean is, the concept of momentum as we define it covers a lot of ground, a wide range of properties is included in the definition. The concept of inertia is more focused. The concept of inertia just says that one thing: that a force is required to change the state of motion of an object.

    If you accept momentum as something meaningful, you're automatically also accepting the concept of inertia. The comprehensive concept of momentum implies the more specific concept of inertia.

    Conversely, the concept of inertia does not specify all of the more general concept of momentum.

    This doesn't mean the word 'inertia' is superfluous. It's practical to have a specific word for an aspect of a larger whole.

    So don't think of one as "the reason" for the other, or one explaining the other. It isn't like that. This is about language. We have concepts, and words to refer to those concepts.
    Last edited: Jun 21, 2012
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