**Speed**

It is the rate of change of displacement with respect to its surrounding. Since the speed of a body is irrespective of its direction, therefore it is a scalar quantity.

**Velocity**

It is also the rate of change of displacement with respect to its surrounding, in a particular direction. Since the velocity is always expressed in a particular direction, therefore it is a vector quantity.

**Acceleration**

It is the rate of change of velocity of a body. It is said to be * positive*; when the velocity of a body increases with time and it is

**when the velocity decreases with time.**

*negative*The negative acceleration is also called

**.***Retardation*