"In Chapter 4, we analyzed the dynamics of a system of particles in terms of the linear momentum about the center of mass of the system and the angular momentum about the center of mass. Un Chapter 5, we examined the kinematics of plane motion, noting that, for a rigid body in plane motion, the angular velocity and angular acceleration vectors must be perpendicular to the plane of motion. This condition, in fact, defines plane motion and greatly simplifies two-dimensional problems. A rigid body has only three degrees of freedom." p428

INERTIA
A PROPERTY OF A BODY THAT RESISTS ANY CHANGE IN MOTION. FOR TRANSLATION MOTION, MASS IS A MEASURE OF THE BODY INERTIA. IT IS HARD TO CHANGE THE STATE OF MOTION OF A HIGH MASS OBJECT. FOR ROTATION, THE MOMENT OF INERTIA OF A BODY IS A MEASURE OF BODY INERTIA.

MOMENT
A FORCE APPLIED TO AN OBJECT AT SOME DISTANCE FROM THE ROTATION AXIS THAT MAY CAUSE THE OBJECT TO ROTATE. THIS IS THE SAME AS A TORQUE. THE MOMENT IS CALCULATED AS THE PRODUCT OF A FORCE TIMES THE DIRSTANCE OF ITS POINT OF APPLICATION FROM THE AXIS OF ROTATION.

ORTHOGONAL
PERPENDICULAR OR NORMAL

ORTHOGONAL AXES
REFERENCE LINES THAT ARE MUTUALLY PERPENDICULAR

PRINCIPAL AXES
THREE SPECIAL AXES OF SYMMETRY OF AN OBJECT USED FOR PREDICTING THE ROTATIONAL BEHAVIOR OF THE OBJECT.

PRINCIPAL MOMENTS OF INERTIA
THESE SPECIFY THE RESISTANCE OF AN OBJECT TO A CHANGE IN ROTATIONAL MOTION WHEN THE PRINCIPAL AXES OF INERTIA ARE USED AS THE REFERENCE AXES.