First, I decided
to use the wheel of the bike as my reference point. They look like older
mountain bike wheels, which I know to be roughly 650mm. I then added 40mm to
account for the extra width of the tire. Finally, the bike is leanings
(although very slightly), which led me to correct for the height in the picture
by a factor of cos(θ). I estimated θ to
be π/24. This all leads me to believe that the vertical height represented by
the wheel in this picture is roughly 684mm.
Using PowerPoint,
I drew one line equal to the height of the wheel, and then stacked the lines to
the top of the can. This gave me an estimate for the tank diameter to be ~3.75 ‘wheels’,
which translates to 2,565mm.
I measured
a can of Campbell’s soup to be 10.8cm high and 6.6cm wide. This gives a Height /
Diameter ratio of 1.63. With this, I scaled my estimate for the diameter to get
a tank height of 4,194mm.
Although we
could estimate the tank’s volume with the volume of the entire tank, I thought
it would be best to incorporate a correction factor. To determine this factor, I
compared the theoretical volume of some commercial storage tanks with their
rated volume. I found that generally, once a tank exceed 300-400L, the
correction factor is roughly 0.98 – 0.99. There is higher variance with smaller
tanks because volume lost to curvature at the ends is relatively meaningful. If
a tank is holding thousands of liters, this curvature is insignificant.
Using a
correction factor of 0.99, I estimate the capacity of this tank to be 21,200 L.
A bit of
research shows that the amount of water required to put out a house fire can
vary wildly depending on the size of the building, how much is (and is not) on
fire, and what other hazards are nearby. There is also no set volume of water
that will put out a fire – any one fire will have some minimum flowrate of
water that must be used to extinguish it. For a small fire, 600 – 800 LPM is typical.
Supposing there
is a 600 LPM fire, our tank would suffice for roughly 35 minutes. Given that
fire fighters typically take 15-20 minutes to put out a house fire, this is
adequate.
Teacher Bird
Once I
started digging into this question, I realized much of the task involved making
and justifying estimate. I chose to include estimates for the bike-lean angle
and the tank correction factor, however even these were pretty insignificant.
It did make the task of organizing my data much more difficult, although the
ability to do this well is extremely valuable.
In all, I
found that I spent almost no time on the use of ratios for this question. I can’t
say how a student would have felt completing this question, but I worry that
they might spend too much time invested in insignificant details (this might also
be a good thing!). Generally, open-ended questions like this serve as excellent
inquiry questions but are probably not best suited for teaching a specific idea
(such as ratios).
There is a
lot of additional geometric complexity one can incorporate in this question.
Suppose we want to use time of day & lengths of shadows to more accurately
determine a reference length? The top of the can is not flat in the picture –
how can we best account for this tilt? Is the ground the bike rests on level? Suppose
we know the required tank thickness – should we use that to determine a
correction factor?