An object is tossed in the air in a parabolic trajectory, with position function $s(t)=-4.9t^2+v_0t+s_0$, where $v_0$ and $s_0$ are the initial position and velocity, respectively, of the object, $s$ is measured in meters, and $t$ is measured in seconds. What is the acceleration of the object? (This magnitude of this quantity is known as $g$ in physics.)

$a(t) =$