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Inertia in rotation

  1. May 8, 2005 #1
    what is the difference between inertia in rotational motion vs. inertia in other motion
  2. jcsd
  3. May 8, 2005 #2


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    They're just two different physical quantities, both of which happen to be conserved. Conservation of linear momentum is a result of the translational symmetry of the laws of physics (ie, the laws are the same here as they are there), while conservation of angular momentum is a result of the rotational symmetry (the laws are the same when your axes are aligned this way as when they are aligned that way).
  4. May 9, 2005 #3
    Everything StatusX said is correct, but the OP asked about inertia.

    The inertia of a particle (inertia = mass) is its resistance to a change in its velocity. This could be a change in direction or speed or both. This is what is meant by the equation a = F/m, or F = ma. For a given applied force, the particle's rate of change of velocity is inversely proportional to its mass (inertia).

    The case for rotation is similar. The rotational inertia of a body measures its resistance to a change in the angular velocity of the body. The rotational inertia depends not only on the mass, but how that mass is distributed about the axis of rotation. It has units of mass x distance^2. The rotational inertia determines how the angular velocity will change as a result of a given applied torque. The more spread out the mass is in relation to the axis of rotation, the more the body resists a change in angular velocity.
  5. Jun 2, 2010 #4
    which are the factors that affect rotational inertia and how are mathematically connected?

    let's say specificaly for disks (with ignorable height) that an axis in their center rotates them
  6. Jun 2, 2010 #5
    What is more difficult to change the rotation of, a lot of mass rotating about an axis it is far away from, or a little bit of mass rotating close to an axis of rotation?
  7. Jun 3, 2010 #6
    let's say that the diameter of the disks is the same

    ofcourse the heavier disk will be more difficult to rotate

    but how much more difficult?
  8. Jun 4, 2010 #7

    Or lets say the masses of the objects that are rotating are exactly the same. but one mass is further away from the axis of rotation than the other..., which is more difficult to change the rotation of, and how much more difficult?
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