## Introducing Rotation Angle and Angular Velocity

In Kinematics, we studied motion along a straight line and introduced such concepts as displacement, velocity, and acceleration. Two-Dimensional Kinematics dealt with motion in two dimensions. Projectile motion is a special case of two-dimensional kinematics in which the object is projected into the air, while being subject to the gravitational force, and lands a distance away. In this tutorial, we consider situations where the object does not land but moves in a curve. We begin the study of uniform circular motion by defining two angular quantities needed to describe rotational motion.

## Rotation Angle

When objects rotate about some axis—for example, when the CD (compact disc) in the figure below rotates about its center—each point in the object follows a circular arc. Consider a line from the center of the CD to its edge. Each **pit** used to record sound along this line moves through the same angle in the same amount of time. The rotation angle is the amount of rotation and is analogous to linear distance. We define the **rotation angle **\(\text{Δ}\theta \) to be the ratio of the arc length to the radius of curvature:

\(\text{Δ}\theta = \cfrac{\text{Δ}s}{r}\text{.}\)

The **arc length**\(\phantom{\rule{0.25em}{0ex}}\text{Δ}s\) is the distance traveled along a circular path as shown in the figure above Note that \(r\) is the **radius of curvature** of the circular path.

We know that for one complete revolution, the arc length is the circumference of a circle of radius \(r\). The circumference of a circle is \(2\pi r\). Thus for one complete revolution the rotation angle is

\(\text{Δ}\theta =\cfrac{2\pi r}{r}=2\pi \text{.}\)

This result is the basis for defining the units used to measure rotation angles, \(\text{Δ}\theta \) to be **radians** (rad), defined so that

\(2\pi \phantom{\rule{0.25em}{0ex}}\text{rad}=\text{1 revolution.}\)

A comparison of some useful angles expressed in both degrees and radians is shown in the table below.

**Comparison of Angular Units**

Degree Measures | Radian Measure |
---|---|

\(\text{30º}\) | \(\cfrac{\pi }{6}\) |

\(\text{60º}\) | \(\cfrac{\pi }{3}\) |

\(\text{90º}\) | \(\cfrac{\pi }{2}\) |

\(\text{120º}\) | \(\cfrac{2\pi }{3}\) |

\(\text{135º}\) | \(\cfrac{3\pi }{4}\) |

\(\text{180º}\) | \(\pi \) |

If \(\text{Δ}\theta =2\pi \) rad, then the CD has made one complete revolution, and every point on the CD is back at its original position. Because there are \(\text{360º}\) in a circle or one revolution, the relationship between radians and degrees is thus

\(2\pi \phantom{\rule{0.25em}{0ex}}\text{rad}=\text{360º}\)

so that

\(1\phantom{\rule{0.25em}{0ex}}\text{rad}=\cfrac{\text{360º}}{2\pi }\approx \text{57.}3º\text{.}\)