The time rate of change of velocity is called acceleration.

Its unit is** m/s ^{2}**. Its dimensional formula is

**[M**. It is a

^{0 }L T^{-2}]**vector quantity**.

Acceleration can be positive, zero or negative.

- Positive acceleration means velocity increasing with time.
- Zero acceleration means velocity is uniform.
- Negative acceleration (retardation)means velocity is decreasing with time.

If a particle is accelerated for a time t_{1} with acceleration a_{1} and for a time t_{2} with acceleration a_{2}, then average acceleration

**Velocity–time graph for motions with constant acceleration:**

In Fig.1 Graph (a) Motion in positive direction with positive acceleration,

Graph (b) Motion in positive direction with negative acceleration,

Graph (c) Motion in negative direction with negative acceleration,

Graph (d) Motion of an object with negative acceleration that changes direction at time t_{1}. Between times 0

to t_{1}, its moves in positive x – direction and between t_{1} and t_{2} it moves in the opposite direction.

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