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Physics 211 Lab #7:

Rotational Dynamics II

A torsion oscillator will be used to investigate the relationship between torque, angular acceleration, and moment of inertia during the pure rotation of an object (see Lectures 18, 19, 20).

Key Activities:

Balancing of Torques on a Torsion Pendulum

In the first activity, the torque exerted by the wire on the torsion pendulum will be studied and calibrated. In particular, by studying the angle Ø at which equilibrium (net torque = 0) is reached between the torque exerted on the pendulum by the hanging masses, 2mgr, and the opposing torque exerted on the pendulum by the wire, kØ, the torsion constant k can be determined, k = 2mgr/Ø (Lectures 18 and 19).

Relationship Between Torque and Angular Acceleration

The relationship between the torque exerted on the pendulum by the wire, and the angular acceleration of the pendulum will be studied in another activity.

Notice that the angular acceleration "follows" the torque (middle and lower graphs), and the ratio, torque/angular acceleration (top graph), is roughly constant and provides an excellent measurement of the moment of inertia, I, confirming the relationship: torque = I * angular acceleration (Lectures 18 and 19).

Energetics of a Torsion Pendulum

The transfer of energy between the kinetic energy of the rotating pendulum, and the potential energy stored in the wire, is illustrated in this activity. Notice that energy is conserved as expected (Lectures 18 and 19).

What's happening?

Oscillatory Period and the Moment of Inertia

An introduction to the relationship between oscillatory period, T, of the torsion pendulum, and the moment of inertia of the pendulum, I, is illustrated in this activity. These results clearly show the expected T ~ (I)^1/2 dependence of the period on the moment of inertia (Lectures 24 and 25).