Average acceleration, a, is defined as the rate of change of velocity, or the change in velocity per unit time. If the initial velocity is +15.0 m/s and 5.0 s is required to slow down to +5.0 m/s, wha...Average acceleration, a, is defined as the rate of change of velocity, or the change in velocity per unit time. If the initial velocity is +15.0 m/s and 5.0 s is required to slow down to +5.0 m/s, what was the car’s acceleration? Average acceleration is the rate of change of velocity, or the change in velocity per unit time. If an automobile slows from +26 m/s to +18 m/s in a period of 4.0 s, what was the average acceleration?