$(a)$ What is acceleration? Write its $SI$ unit.
$(b)$ Draw velocity-time graphs for an object having:
$(i)$ Uniformly accelerated velocity.
$(ii)$ Uniformly retarded velocity.

(N/A) Acceleration is defined as the rate of change of velocity of an object with respect to time. Mathematically,$a = \frac{v - u}{t}$. Its $SI$ unit is $m s^{-2}$.
$(b)$ The velocity-time graphs are as follows:
$(i)$ For uniformly accelerated velocity,the graph is a straight line sloping upwards from the origin (or starting from a non-zero initial velocity),indicating that velocity increases at a constant rate over time.
$(ii)$ For uniformly retarded velocity,the graph is a straight line sloping downwards,indicating that velocity decreases at a constant rate over time until it reaches zero.