We begin by considering a system of n point masses m1, m2, . . . , mn situated at perpendicular distances d1, d2, . . . , dn from a straight line L . Then the quantity
ILm1d 2 1m2d
2 2 . . .mnd
is called the moment of inertia of the system of particles about L . If M/ml/ m2/. . ./mn is the total mass of the n particles, the length kL defined by
or equivalently by
is called the radius of gyration of the particles about the line L .