Mechanics

Kinematics

Average velocity

$$\bar{\upsilon}=\frac{\Delta x}{\Delta t}$$

Velocity

$$!\mathbf{v} = \frac{d \mathbf{x}}{dt}$$

or

$$!\mathbf{v} = \mathbf{i}\frac{d{x}}{dt} + \mathbf{j}\frac{d{y}}{dt} + \mathbf{k}\frac{d{z}}{dt}$$