1. Rotational Inertia
8.2

2. TEKS 2C, 3A, 4B, 4C

4. Center of Mass
Center of mass – point at which all the mass of the body can be considered to be concentrated when analyzing translational motion.
For a free body in which the only force acting on it is gravity, the point around which the object will rotate.
Animation:

5. Rotational and translational motion can be combined
Stick rotate about its center of mass.
The center of mass moves as a point mass; moves as a projectile.

6. Moment of Inertia
Moment of inertia – a measure of the object’s resistance to a change in its rotational motion.
The rotational analog of mass
Depends on the distribution of the mass.
Calculating the moment of inertia
In general, the further a given mass is from the axis of rotation, the more difficult it is to change the rotational motion.
Animation:

7. Note that for a given mass at a given radius from the axis of rotation, it does not matter how the mass is distributed.
Units for moment of inertia (I ): kg•m2

8. Rotational Equilibrium
Equilibrium requires zero net force and zero net torque.
F = 0
 = 0
Second condition for equilibrium – dependence of equilibrium on the absence of torque.
Animation:

9. Practice 8B, #s 1 and 3
Section 8.2 Review, p. 289