You want to find the moment of inertia of a complicated machine part about an axis through its center of mass. you suspend it from a wire along this axis. the wire has a torsion constant of 0.490 n⋅m/rad . you twist the part a small amount about this axis and let it go, timing 185 oscillations in 225 s .

The time of a torsional harmonic oscillator is computed by:2 pi sqrt ( I / kappa ), where:I is the moment of inertia;and kappa is the torsion constant (N m per radian); and I is the moment of inertia in kg m^2; 
Since Nm = kg m^2 / s^2, the period is in seconds. 
In the case at hand, the period is (225/185) seconds, so 
(225/185) s = 2 pi sqrt( I / 0.490 N m ) 1.2162 s / 2pi = sqrt( I / 0.490 N m) (0.1936 s)^2 = I / 0.490 N m I = (0.1936)^2 (0.490) kg m^2 = (0.0375) (0.490) kg m^2= 0.0184 kg m^2

Answer: fn = wcos50 = 38.89(9.81)(cos50) = 245.23 n

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