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What the heck is inertia anyway?

  1. May 15, 2005 #1
    HI, I never really gave it much thought, but does anyone have an explanation for inertia at the molecular or atomic level. I think that it takes time for the force or energy to travel throughout the molecules and/or atoms of a body and that is why it continues to stay in motion. Can anyone explain it better, or more correct.
  2. jcsd
  3. May 16, 2005 #2

    James R

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    Nobody really knows why inertia exists. Inertia is quantified by the thing we call "mass", which gives the resistance of an object to being accelerated (a = F(net)/m).

    Why should things with mass resist being accelerated? That is a complete mystery. An equivalent question is simply "Why does mass exist?", and nobody knows the answer to that one, either.
  4. May 16, 2005 #3
    Hi quasi,
    The principle of relativity says that the laws of physics are the same in a constantly moving reference frame. This is why you can do a physics experiment in the lab and analyse it without having to take into account the motion of the lab around the Earth, or Earth around the Sun, or the Sun around the galaxy, and so on.

    This means that the tendency for an object to stay in motion is exactly the same as the tendency for an object to remain at rest... there is no real difference between the two. So, if you're happy to accept that an object at rest should tend to remain at rest, then the inertia of an object in motion needs no further explanation.

  5. May 16, 2005 #4
    I was always taught this definition :

    Inertia is the property of matter to resist accleration or deceleration, where accelertation and deceleration are of course, any motion which is not in a straight line and with constant velocity.

    Which made me think, is inertia the resistance of a bodyto have unbalanced forces acting upon it? As to turn an object, or to stop it having constant velocity, one must change the forces acting on it.


  6. May 16, 2005 #5
    Maybe you just chose your words poorly but just in case:
    If some forces act upon an object it does not move at a constant velocity or unless the forces cancel eachother out.
  7. May 16, 2005 #6
    If you want a definition of the moment of inertia I when doing rigid-body physics, here it is : the I expresses how mass is distributed over the entire object's volume. When a rigid body (thus no point particle) moves down, the acceleration does not depend on the mass, the volume but the way mass is distributed over the volume. When you are going downhill with your bicycle, you will go faster if you lean over, right (let's neglect air resistance)? Well, you know the reason.

    The I is also called mass-tensor and i am sure you all know what a tensor is and why it is useful in physics

  8. May 17, 2005 #7
    Inertia is that property of a body which resists changes in momentum.

  9. May 23, 2005 #8
    Hey just think that inertia is just mass

    Hey there ! What i mean is that inertia is something that is used in rotational motion and mass is used in place of inertia in translational motion. Ex. F=m.a(i.e. For translational motion) and Tau=inertia . alpha(for rotational motion) . For mass we usually use inertia in rotational dynamics.
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