21.1.2—Equations of the form x = x0 sin ωt representing a sinusoidally alternating

A sinusoidal alternating quantity is modelled by x=x₀sin(ωt+φ), where x₀ is peak value and φ sets the phase.

Use the initial value and slope to choose phase; differentiate or inspect the graph to identify when the signal is increasing or decreasing.

A signal starting at zero and rising has φ=0 in x=x₀sinωt.

The sine expression gives instantaneous value, not rms value or average magnitude.