$2.$ A harmonic motion is expressed by the relation x $=0\mathrm{.}5$ sin $(10$t $+4),$ where x is expressed in inches, t in seconds, and the phase angle in radians. Determine:

$($a$)$ Circular and natural frequencies of vibration, and

$($b$)$ Displacement, velocity, and acceleration at $0\mathrm{.}3$ sec$.$