Two graphs for the motion of objects moving along a straight line are shown. State how the speed is changing with time in both cases.

(N/A) Case $1$: The graph is a horizontal straight line parallel to the time axis. This indicates that the speed of the object remains constant at $20 \text{ m s}^{-1}$ over time.
Case $2$: The graph shows that the speed is changing continuously with time. The speed increases from $0$ to $20 \text{ m s}^{-1}$ between $0$ and $2 \text{ s}$,then decreases to $10 \text{ m s}^{-1}$ at $3.5 \text{ s}$,and finally increases again. Thus,the object is moving with variable speed.