Does a cycle (in Simple Harmonic Motion) have to equal 2π?

Question

So, I search for the definition of cycle and I get this in Wikipedia:

A turn is a unit of angle measurement equal to 360° or 2π radians (or ...). A turn is also referred to as a revolution or complete rotation or full circle or cycle or rev or rot.

When I look for motion in pendulum, I get:-

The time for one complete cycle, a left swing and a right swing, is called the period.

So, in the first quote, it says that one cycle has to be 2π radians, but in the second quote it says "one complete cycle" is "a left swing and a right swing" (i.e the full cycle doesn't have to be equal to "2π radians").

Can you please clarify what I see as contradicting statements?

Answer 1

The $2\pi$ refers to the phase angle ($\phi $), whereas in a pendulum, we have an additional, physical angle (angle of inclination $\theta$). In a pendulum of small oscillations of maximum angle of inclination $\theta_0$, the two are related by $\theta=\theta_0\sin\phi$. Note that $\phi=\phi_0+\omega t$ is a time-dependent quantity.

Simply put, this phase angle denotes the state of a periodic system in one variable. The system repeats every time $\phi$ increments by $2\pi$. An angle is used because simple harmonic oscillators are sinusoidal and can be represented on a circle. In this imaginary circle, one 'turn' is $2\pi$ change in $\phi$. It is simultaneously a cycle in the pendulum.

Remember that definitions die when taken out of context ;)

Answer 2

The two quoted definitions are both right but they apply to different contexts.

A general definition of a "cycle" is the minimal time after which the system returns to the same configuration. For a rotating object (e.g., a pendulum turning in one direction only) , the cycle is a full turn. For an oscillating pendulum, the cycle is a single "there-and-back" tour.