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For a simple pendulum, this definition yields a formula for the moment of inertia I in terms of the mass m of the pendulum and its distance r from the pivot point as, =. Thus, the moment of inertia of the pendulum depends on both the mass m of a body and its geometry, or shape, as defined by the distance r to the axis of rotation.
A compound pendulum (or physical pendulum) is one where the rod is not massless, and may have extended size; that is, an arbitrarily shaped rigid body swinging by a pivot . In this case the pendulum's period depends on its moment of inertia I O {\displaystyle I_{O}} around the pivot point.
The moments of inertia of a mass have units of dimension ML 2 ([mass] × [length] 2). It should not be confused with the second moment of area, which has units of dimension L 4 ([length] 4) and is used in beam calculations. The mass moment of inertia is often also known as the rotational inertia, and sometimes as the angular mass.
The pendulum carries an amount of air with it as it swings, and the mass of this air increases the inertia of the pendulum, again reducing the acceleration and increasing the period. This depends on both its density and shape. Viscous air resistance slows the pendulum's velocity. This has a negligible effect on the period, but dissipates energy ...
The pendulum is assumed to consist of a point mass, of mass , affixed to the end of a massless rigid rod, of length , attached to a pivot point at the end opposite the point mass. The net torque of the system must equal the moment of inertia times the angular acceleration:
Double compound pendulum Motion of the double compound pendulum (from numerical integration of the equations of motion) In a compound pendulum, the mass is distributed along its length. If the double pendulum mass is evenly distributed, then the center of mass of each limb is at its midpoint, and the limb has a moment of inertia of I = 1 / ...
Cavendish's diagram of his torsion pendulum, seen from above. The pendulum consists of two small spherical lead weights (h, h) hanging from a 6-foot horizontal wooden beam supported in the center by a fine torsion wire.
The pendulum is usually adjusted by moving the moment of inertia adjustment weights towards or away from the centre of the mass by equal amounts on each side in order to modify f R, until the rotational frequency is close to the translational frequency, so the alternation period will be slow enough to allow the change between the two modes to ...