Acceleration refers to the process of an object changing its speed, whether by increasing or decreasing speed, or changing direction. An object accelerates when its speed increases over time. An object also accelerates when its speed decreases, called deceleration or negative acceleration. Even objects moving at a constant speed accelerate if they change direction, like seats on a Ferris wheel. To calculate acceleration, you determine the change in speed over a period of time, with acceleration measured in meters per second squared.