The velocity is a vector quantity and is the speed with a direction. Acceleration exists when there is a change in speed–either by speeding up or slowing down–or change in direction or both changing simultaneously. The mathematical way of expressing acceleration is the change in velocity over time. Assume an object is moving at a constant speed. While making a turn, it slows down. Its instantaneous velocity changes both in magnitude and direction. Let the instantaneous velocity vectors located at P1 and P2 be vectors v1 and v2. By subtracting the vectors, the change in velocity, delta vector v, is obtained. Now, the direction of the average acceleration vector will be in the direction of the change in velocity. The average acceleration vector is given by the ratio between the Δ vector v and Δt. When we consider smaller intervals of time, the position P2 approaches P1. At this instant, as Δt approaches zero, the average acceleration approaches instantaneous acceleration.