If you think of frequency in Hertz as cycles per second, all the 2π you find in these equations convert Hz to “radian frequency” since one cycle per second is really 360 degrees of the sine ...

Thus, radian measure is a ratio of two lengths. For the sine wave in Figure 3.2a, we know that a maximum occurs at 90°. This corresponds to a radian measure of Jt/2 radians. The first complete cycle ...