To accelerate means to speed up. The greater the **acceleration**, the greater the change in velocity over a given time. The **average acceleration** is the rate at which velocity changes:

\[a_{avg}=\dfrac{\Delta v}{\Delta t}=\dfrac{v_f-v_0}{t_f-t_0}\]

where \(v\) is velocity, and \(t\) is time. Acceleration is a vector in the same direction as the change in velocity, \(\Delta v\). Since velocity is a vector, it can change either in magnitude or in direction. Acceleration is, therefore, a change in either speed or direction, or both. The “acceleration due to gravity” is also equal to the “gravitational field”. Equivalence of inertial and gravitational mass.