My textbook defines radius of gyration as

The radius of gyration of a body about an axis may be defined as the distance from the axis of a mass point whose mass is equal to the mass of the whole body and whose moment of inertia is equal to the moment of inertia of the body about the axis.

Consider a conical pendulum with a bob of mass m and a massless string of length L, having an angular displacement of θ with the vertical axis. enter image description here

The mass of the bob is concentrated at the centre of the bob and the axis is AB. Might be a silly question but will the radius of gyration be L or Lsinθ? Does the definition refer the "distance" as "displacement"? or am i flawed in some other way too?

  • $\begingroup$ Please see this. $\endgroup$
    – joseph h
    Jan 21 at 5:43


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