In general, when a body is moving, its velocity is not always the same. A body whose velocity is increasing is said to be accelerated. Average acceleration is defined as change in velocity divided by the time interval. Let us consider the motion of a particle. Suppose that the particle has velocity vector v1 at t = t1 and at a later time t = t2 it has velocity vector v2. Thus, the average acceleration during time interval Δt = t2 - t1 is
If the time interval approaches to zero, average acceleration is known as instantaneous acceleration. Mathematically,
