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Homework Help: Do I understand this correctly?

  1. Jan 5, 2015 #1
    1. The problem statement, all variables and given/known data
    the theory behind Parallel axis theorem

    2. Relevant equations
    parallel axis theorem:Io=Ig+md²
    Radius of Gyration=Ig=m*K²
    3. The attempt at a solution

    ok folks If I understand this theory correctly the radius of gyration is the radius or distance from the axis of rotation to the center of the AREA

    the parallel axis then takes this area into account and converts the equation to account for the area and the mass.

    I am trying to teach myself dynamics before I pay for another course at college and bomb out because the professors dont explain anything...

    is this a correct understanding? if not can you help me?
  2. jcsd
  3. Jan 5, 2015 #2
    sorry, not center of area, but center of mass

    ill need help understanding this as well

    Last edited: Jan 5, 2015
  4. Jan 5, 2015 #3


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    Staff Emeritus
    Science Advisor
    Homework Helper

    It often helps to know some definitions when trying to teach yourself a subject:


    Insofar as a connection between the gyradius and the parallel axis theorem, there is only an indirect one.

    The PAT is used to calculate the MOI about a different set of axes, given an inertia value about an axis which is parallel. For example, when calculating the MOI of a composite area or mass, we usually want the MOI about the centroid of the combined area or mass. The PAT allows us to make this calculation, using the area or mass, the location of the centroid of each part, and the MOIs of the individual parts.

    The gyradius, on the other hand, gives a rough measure of the distribution of mass or area about an axis. A larger gyradius implies that the area or mass is distribued further away from the axis. The gyradius would typically be calculated for a composite area or mass after its MOI about the centroid had been determined.
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