**PURPOSE: **To illustrate the rotational analog of Newton's second law of motion.

**DESCRIPTION: **Various masses *M* can be hung on a string wound around an axel of either of two radii *R* to provide a torque *T* = *M*g*R*. Four brass masses *m* can be positioned along the arms at one of four distances* l* from the axis of rotation to provide a moment of inertia *I* = 4*ml*^2. The angular acceleration* a = T/I* =* M*g*R*/4*ml*^2 can then be calculated. The angular acceleration can be determined experimentally by measuring the time required for the system to rotate one complete revolution starting from rest: *a* = 2 *d*/*t*^2, where* t* is the time required for the device to rotate through the angle *d* in radians.

**SUGGESTIONS: **

**REFERENCES: ** (PIRA 1Q20.20) See Demonstration Reference File for further information, including sample data.

**EQUIPMENT: **Oberbeck Cross with weights and manual timer.

**SETUP TIME: **None.