In physics and classical mechanics, acceleration is formally defined as the vector rate of change of an object's velocity with respect to a change in time. Because velocity incorporates both the speed and the directional trajectory of an object, an acceleration can occur due to a change in magnitude, a change in direction, or a combination of both. Mathematically calculated as the derivative of velocity over time, it is measured in meters per second squared and is driven universally by the net forces acting upon physical masses according to Newton's laws of motion.