When a

particle's velocity changes, the

particle is said to undergo

acceleration (or a change in

velocity). For motion along an

axis, the

average acceleration *Aavg* over a

time interval *t* is:

*Aavg* = v2 - v1 / t2 - t1 = v/t

where the particle has velocity v1 at time t1 and then velocity v2 at time t2. The instantaneous acceleration (or simply **acceleration**) is the derivative of the velocity with respect to time.

In other words, the acceleration of a particle at any instant is the rate at which its velocity is changing at that instant. Graphically, the acceleration at any point is the slope of the curve *v(t)* at that point.