Acceleration is a vector quantity that is defined as the rate at which an object changes its velocity. An object is accelerating if it is changing its velocity.
Since acceleration is a vector quantity, it has a direction associated with it. The direction of the acceleration vector depends on two things:
- whether the object is speeding up or slowing down
- whether the object is moving in the + or – direction
The general principle for determining the acceleration is that if an object is slowing down, then its acceleration is in the opposite direction of its motion.
c1520: hasten the occurrence of
c1590: make quicker; to hasten, quicken; make haste
c1640: go faster, become faster