The Speed
Differentiating Eq. 11-17 with respect to time - with 
held constant - leads to
However, 
is the linear speed (the magnitude of the linear velocity) of the point in question, and 
is the angular speed 
of the rotating body. So
(radian measure). 11-18
Caution: The angular speed must be expressed in radian measure.
Equation 11-18 tells us that since all points within the rigid body have the same angular speed , points with greater radius have greater linear speed 
. Figure 1 l-9a reminds us that the linear velocity is always tangent to the circular path of the point in question.
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