Let us discuss the definitions of speed, uniform speed, variable speed, average speed and instantaneous speed in the following section:

**1. Speed:** The time rate of change of position of a body in any direction is known as its speed.

It is measured by the ratio of the distance by a body to the time in which this distance is covered

i.e. Speed = distance/time

Speed is a scalar quantity. It is either positive or zero but never negative.

Units of speed: We know that,

Speed = Distance/Time

Units of speed = Units of distance/Units of time

In the CGS system, unit of speed is cms^{-1} and in the MKS system or SI units the unit of speed is ms^{-1}.

**2. Uniform Speed:** A body is said to be moving with uniform speed, if it covers equal distances in equal intervals of time, however small these intervals may be.

**3. Variable Speed:** A body is said to be moving with variable speed if it covers unequal distances in equal intervals of time, however small these intervals may be.

**4. Average Speed:** Average speed of a body is defined as that uniform speed with which it covers the same distance in a given interval of time as is actually covered by it in the same interval of time while moving with variable speed.

It is given by the ratio of the total distance traveled by the body to the total time taken by it.

i.e. Average speed =Total distance traveled/Total time taken

**5. Instantaneous Speed:** When a body is moving with variable speed, then, its speed at any instant of time is known as its instantaneous speed.

If a body travels a very small distance Ds in a very small interval of time Dt, then its

Instantaneous speed = limit time tends to 0(ds/dt)