What does the dot over $x$ or $y$ mean?
It usually means derivative with respect to time. The notation goes back to Newton.
By the way, $\ddot{x}$ is often used in Physics for the second derivative of $x$ with respect to time.
It usually means derivative with respect to time. The notation goes back to Newton.
By the way, $\ddot{x}$ is often used in Physics for the second derivative of $x$ with respect to time.