$\bullet$ If the displacement of an oscillator in SHM is described by the equation $y=(0.25 \mathrm{~m}) \cos [(314 \mathrm{rad} / \mathrm{s}) t],$ where $y$ is in meters and $t$ is in seconds, what is the position of the oscillator at (a) $t=0,$ (b) $t=5.0 \mathrm{~s},$ and $(\mathrm{c}) t=15 \mathrm{~s} ?$