Acceleration is the time rate at which a velocity is changing.
This condition means that the acceleration of a point moving on a curved path can never be zero. The average acceleration during a time interval is equal to the total change in the velocity during the interval divided by the time interval. The acceleration at any instant is equal to the limit of the ratio of the velocity change to the length of the time interval, as the time interval approaches zero. If velocity is stated in metres per second, acceleration will be stated in metres per second per second.