In the context of linear motion, the direction of the object in motion does not change; but the magnitude of the velocity may change.

When the velocity of an object changes, the object is said to be accelerating.

**Acceleration**is defined as the

**rate of change in velocity**

**.**

Acceleration is a vector quantity. SI unit for acceleration, a is ms

^{-2}, metre per second squared.__Positive acceleration__

If the velocity of an object increases from an initial velocity, u, to a higher final velocity, v.

The acceleration of an object is uniform if its velocity increases at a uniform rate.

__Negative acceleration is called deceleration.__

If the velocity of an object decreases from an initial velocity, u, to a lower final velocity, v, its acceleration is negative.

The deceleration of an object is uniform if its velocity decreases at a uniform rate.

__Zero acceleration__

An object moving at a constant velocity, that is, the magnitude and direction of its velocity remain unchanged - is not accelerating; its acceleration is zero.

Quiz :

An object accelerates at a constant rate from rest and reaches a velocity of 10 ms-1 after 5.0 s.

What is it acceleration?

Tips :

**rest**mean stationary ( zero velocity )