Why do this problem?
requires careful thought about the way the water level
in a vessel changes when water is added at a constant rate. Through
analysing the key features of a graph, students can figure out the
shape of the vessel it represents.
The first part involves working out volumes. The key is to
realise that the cross-sectional area is proportional to the volume
and then to work out the areas. There are obvious 'easy' candidates
for this and some harder letters. There are various ways in which
the areas of the cross sections of the vessels can be 'rearranged'
to form rectangles. Students could work on finding the areas in
small groups and then feed back to the rest of the class, sharing
their approaches for finding the trickier areas.
To work out which letter the graph corresponds to, ask for
suggestions for a 'story' relating the height-chart diagram to a
vessel filling up. For example, what happens to the water level at
the horizontal parts of the graph? What could be happening to
account for this?
Once the class have identified the correct vessel for the
graph, they could work on producing graphs for the other letters.
Students could check each other's work by seeing if they can match
the graphs with the vessels.
Small groups of students could also design some other letters
in the same way and draw the resulting graphs, perhaps producing a
card-matching activity to challenge other groups. The results could
contribute to a classroom display.
What could be happening at the horizontal parts of the
What can you work out from the steepness of the lines on the
Would the graphs change if the holes were moved, or if water
was poured into both holes where available?
The final part of the M graph should be a curve rather than a
straight line. Can students justify why the graphs for V, A, and S
will also contain curves?
Can students work out the functions which describe any of
these curved parts?
Start by working on the letters without diagonal lines and
work out how quickly they will fill up.