### Rotational Kinetic Energy

The rotational kinetic energy of a rigid assembly of masses (or mass distribution) is the sum of the rotational kinetic energies of the component masses. Therefore, consider a point-mass rotatingB.13 in a circular orbit of radius and angular velocity (radians per second), as shown in Fig.B.4. To make it a closed system, we can imagine an effectively infinite mass at the origin . Then the speed of the mass along the circle is , and its kinetic energy is . Since this is what we want for the rotational kinetic energy of the system, it is convenient to define it in terms of angular velocity in radians per second. Thus, we write

 (B.7)

where

 (B.8)

is called the mass moment of inertia.
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Translational Kinetic Energy