Consider the motion of a point.

Assuming a system of rectangular axes, let $x$, $y$, and $z$ denote the general rectangular coordinates of the point at the general time t.

We can denote the velocity of the point in each axis via the quantities $\frac{\mathrm{d}x}{\mathrm{d}t}$, $\frac{\mathrm{d}y}{\mathrm{d}t}$, $\frac{\mathrm{d}z}{\mathrm{d}t}$.

Input the derivative for the velocity of the point in each axis using $\frac{d}{dt}$.