Simple Harmonic Motion can be defined in two equivalent ways. One by the displacement equation x(t) = A cos(ωt + φ). What is the other equivalent way?
The context states: 'To summarise the discussion so far, simple harmonic motion can be defined in two equivalent ways, either by Eq. (13.4) for displacement or by Eq. (13.13) that gives its force law.' Eq. (13.13) is F(t) = -kx(t). (NCERT, page 267)