Explain why we can only calculate a stone's average speed during its fall.

Diagrams in are distance—time graphs for four moving objects. Complete the table by indicating (in the second column) the graph or graphs that represent the motion described in the first column.

Sketch a distance-time graph for the car whose journey is described here.
• The car set off at a slow, steady speed for $20 \mathrm{s}$. 
• Then it moved for $40 \mathrm{s}$ at a faster speed.
• Then it stopped at traffic lights for $20 \mathrm{s}$ before setting off again at a slow, steady speed.