So for an object in simple harmonic motion the value of the phase angle \(\phi\) tells you the point in the oscillation cycle that corresponds to \(t = 0\). Note that Figures 12-6a and 12-6b really depict the same oscillation, with the same amplitude \(A\) and period \(T\); the only difference is the point in the cycle that we call \(t = 0\).
The phase angle also appears in the expressions for velocity and acceleration in simple harmonic motion (Equations 12-7 and 12-8), which we found by analogy with uniform circular motion: