A solid body rotates about a stationary axis so that its angular velocity depends on the rotation angle \(\varphi\) as \(\omega=\omega_{0}-a \varphi,\) where \(\omega_{0}\) and \(a\) are positive constants. At the moment \(t=0\) the angle \(\varphi=0\). Find the time dependence of (a) the rotation angle; (b) the angular velocity.