If the magnitude or the direction or both of the velocity changes with respect to time, the particle is said to be under acceleration.

Acceleration of a particle is defined as the rate of change of velocity. Acceleration is a vector quantity.

**Acceleration = change in velocity / time taken **

If u is the initial velocity and v, the final velocity of the particle after a time t. then the acceleration.

**a = (v – u)/t**

Its unit is ms^{-2} and its dimensional formula is LT^{-2}.

The instantaneous acceleration is, **a = dv/dt = d/dt (ds/dt) = (d ^{2}s/dt^{2})**.