The Torsion Pendulum

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Pendulum Moment of Inertia

The pendulum has mass, and, therefore, its own moment of inertia.  When we make a measurement using the pendulum, we are actually measuring the moments of inertia of both the gun and of the pendulum.  To arrive at the correct value for the gun, alone, we subtract the pendulum's moment of inertia. Since the pendulum has much less mass (it weighs much less) than a typical gun, and since the mass is fairly well concentrated about the pendulum's balance point, values for the pendulum moments of inertia are substantially smaller than those typical of shotguns. The pendulums I have made have had moment of inertia values ranging from about               0.014 slug-ft^2 to 0.017 slug-ft^2.  These values are about 10 percent of the value of a typical game gun.