Motion in Straight Line

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Speed is defined as the distance travelled by a body per unit time. 
Thus,
$$\begin{gathered} \mathrm{Speed} = \frac{\mathrm{Distance} \mathrm{travelled}}{\mathrm{Time} \mathrm{taken}} \\ \boxed{\mathrm{v} = \frac{\mathrm{s}}{\mathrm{t}}} \end{gathered}$$

The SI unit of speed is ms-¹ whenever the distance covered by the body is expressed in metre and time in second.
A smaller unit of speed is cm s^-1.
A larger unit of speed is km h^-1. 
Speed is a scalar quantity because it has only magnitude and no direction. 

The speed of a body tells us how fast or how slow the body is moving. But it does not give the direction of motion.

Dimension formula : [M⁰L¹T^-1]

Speed can be two types uniform speed and non-uniform speed.

In uniform speed, a particle covers equal distances in equal intervals of time.

In non-uniform speed particle covers unequal distances in equal intervals of time.

Average speed:

Average speed is defined as the total path length travelled divided by the total time interval during which the motion has taken place.

$Average speed = \frac{Total path length}{Total time interval}$

Average Velocity:

Average velocity is defined as the change in position or displacement (∆x) divided by the time intervals (∆t).

$\overline{v} = \frac{x_2 -x_1}{t_2- t_1} = \frac{∆x}{∆t}$

where x₂ and x₁ are the positions of the object at time t₂and t₁, respectively.

The bar over the symbol for velocity is a standard notation used to indicate an average quantity. The SI unit for velocity is m/s or ms^–1.