Detailed example
Why do babies dehydrate faster than adults in summer?
This example demonstrates how the different outcomes and areas of study can be taught together using content from Area of Study 4: Space and measurement that is also supported by Area of Study 1: Algebra, number and structure. Outcomes provide the key knowledge and skills (Outcome 1), that are taught using a mathematical investigation (Outcome 2). This can be supported by the use of technology (Outcome 3) in developing the report on the research and investigation.
It poses a relatively complex, conceptual challenge for students to come to grips with, and the use of hands-on materials supports and enables this to happen.
Materials required:
- A set of interlocking centimetre cubes – enough for students to work in pairs or small groups using the cubes.
Mathematical content covered
In this investigation and activity, the focus is on the problem-solving process (Outcome 2) and on using and applying mathematical content taken from both Area of Study 1: Algebra, number and structure and Area of Study 4: Space and measurement in Outcome 1.
Outcome 2: Key knowledge
- uses and applications of mathematics and numerical data and information in aspects of contemporary life and the embedded nature of this mathematics in work, social and personal contexts
- relevant and appropriate mathematics in areas relating to student’s study, work, social or personal contexts
- common methods of presenting and communicating mathematics in everyday life, for example charts, graphs, maps, plans, tables, algebraic expressions and diagrams.
Outcome 2: Key skills
- identify and recognise how mathematics is used in everyday situations and contexts, making connections between mathematics and the real world
- extract the mathematics embedded in everyday situations and contexts and formulate what mathematics can be used to solve practical problems in both familiar and new contexts
- represent the mathematical information in a form that is personally useful as an aid to problem-solving, such as a table, summary, chart, numeric or algebraic representation, physical model or sketch
- undertake a range of mathematical tasks, applications and processes to solve practical problems, such as drawing, measuring, counting, estimating, calculating, generalising and modelling
- interpret results and outcomes of the application of mathematics in a context, including how appropriately and accurately they fit the situation, and to critically reflect on and evaluate the mathematics used and the outcomes obtained relative to personal, contextual and real-world implications
- represent, communicate and discuss the results and outcomes of the application of mathematics in a range of contexts.
Outcome 1, Area of Study 1: Algebra, number and structure
Key knowledge
- conventions of formal mathematical terminology and notations in calculations, symbolic expressions and formulas
- algorithmic, algebraic and computational strategies
- number facts, operations and relationships for calculations
- ratios, proportions and percentages
- manipulation of formulas
- estimations and approximations
- contextual and real-world meaning of numerical results.
Key skills
- use and apply the conventions of mathematical notations, terminology and representations
- make estimates and carry out relevant calculations using mental and by-hand methods
- use different technologies effectively for accurate, reliable and efficient calculations
- solve practical problems which require the use and application of a range of numerical and algebraic computations
- solve practical problems requiring graphical and algebraic processes and applications, including substitution into formulas
- use estimation and other approaches to check the outcomes, including for accuracy and reasonableness of results
- evaluate the mathematics used and the outcomes obtained relative to personal, contextual and real-world implications.
Outcome 1, Area of Study 4: Space and measurement
Key knowledge
- the names and properties of shapes and objects and their representations
- metric and other related quantity measures
- routine digital and analogue measurement tools and instruments and scales
- formulas for calculating length, area, surface area, volume and capacity
Key skills
- interpret and describe shapes and objects and their representations using geometric and spatial language and conventions
- calculate and interpret area, surface area, volume, capacity and density of routine, including compound, shapes
- use estimation, rounding and approximation strategies to check the outcomes and interpret the results and to reflect on the outcomes obtained relative to personal, contextual and real-world implications.
Identifying the context for the investigation
Every summer there are news reports about babies or young children being left in cars in the hot weather, with often dire consequences. In Australia, there can be in excess of 5000 reports of children needing to be rescued after being left unattended in a car.
Leaving children unattended in a car – even for a short period of time – can be fatal. Children are particularly at risk because they can lose fluid quickly, become dehydrated and suffer from heatstroke.
For information about this situation and the recommended actions, visit
Kid Safe Victoria and look under the Safety Advice section for further information. The information provides advice about what parents or providers need to do to make sure this doesn’t happen.
Formulate the question or problem to be investigated
Why do babies dehydrate faster than adults in summer? What is the mathematics sitting behind this? How can you begin to investigate this issue? Brainstorm with students the factors that might be relevant. What are some differences between babies and adults? Include physical attributes. Why might this be relevant? Introduce the idea of dehydration being related to the amount of skin (surface area).
Think about possible problem-solving strategies that you could utilise, and propose and work through these questions:
How could you model the situation?
How could you take:
- a simpler case
- a simpler shape – after all, people are very complicated shapes
- and a simple size – say the adult is twice the scale of the baby.
Suggest the use of centimetre cubes and ask how you could create a simple model of an adult versus a baby using the cubes.
Explore the problem mathematically
Agree on how you might work through this logically and develop a process for investigating the relationship between length, surface area and volume for a baby versus an adult, represented by the cubes. The following notes give you a suggested approach to follow, using the blocks to model and research what happens as you model a doubling of size, then what happens if you treble the relative size of the adult model.
Doubling …
Use 3 small cubes to make a model of your baby.
- What is its volume?
- What is its surface area (skin), i.e. how many little squares would cover it?
Now make a model of an adult that is double in scale.
- What is its volume?
- What is its surface area (skin)?
Complete the table with the outcomes:
Dehydration 1 |
length |
surface area |
volume |
---|
original model ... baby | | | |
model doubled in scale ... adult | | | |
Did the surface area and volume increase by the same amount, or did one increase more than the other?
- How many square units of skin for each cubic unit of volume allow water to be lost:
- by the baby?
- by the adult?
- so, which would lose water faster?
But why?
And is an adult only double the size of a baby? Should we model a bigger sized adult?
So, we have the beginning of an answer, but let's try to go a bit further to understand what's happening. Fill in the table below, by continuing to double the scale.
Can you see a pattern that lets you work out the last row or two without having to actually build the model?
Dehydration 2 |
length |
surface area |
volume |
---|
original model ... baby | | | |
model doubled in scale ... adult | | | |
model doubled in scale again ... | | | |
and again ... | | | |
and again ... | | | |
Why does the area go up by a factor of 4? and the volume by a factor of 8?
Look at the 2 models
Each single unit of skin in the little one:
- is replaced by 4 units of skin in the larger one,
- i.e. a 1x1 square is replaced by a 2x2 square.
Each single unit of volume in the little one:
- is replaced by 8 units of volume in the larger one,
- i.e. a 1x1x1 cube is replaced by a 2x2x2 cube.
Discussion and extension
But would this be true for shapes other than our simple one?
Take a slightly more complex shape and investigate.
What happens when we double this shape in scale? Try it ...
But what if it's not just doubling?
We have begun to see what's happening when simple things are doubled in scale … but, an adult probably isn't simply double the scale of a baby – we just assumed that at the beginning to make life easier.
Can we predict what will happen if the adult is 3 or 10 or 100 times the scale of the baby?
Use the blocks to treble the model.
Then try to use the pattern to fill in the rest of the table.
Dehydration 3 |
length |
surface area |
volume |
surface area
÷ volume |
---|
original model ... baby | | | | |
model doubled in scale ... adult | | | | |
model doubled tripled in scale | | | | |
model x 10 | | | | |
model x 10 | | | | |
The last column gives a measure of how many units of available skin are able to lose water compared to each unit of volume.
So, what does this mean about dehydration when the scaling factor is 3?
Again, look at the two models.
Each single unit of skin in the little one:
- is replaced by 9 units of skin in the larger one,
- i.e. a 1x1 square is replaced by a 3x3 square.
Each single unit of volume in the little one:
- is replaced by 27 units of volume in the larger one,
- i.e. a 1x1x1 cube is replaced by a 3x3x3 cube.
So, as we treble, which is growing faster, the skin or the volume?
Which then, of these people, would dehydrate fastest?
Formalising our findings and refining the model
Can we connect this with what we know about calculating surface area and volume for such shapes? Can we use the formulas for calculating surface area and volume of the above shapes (rectangular prisms) to analyse the above question, now we have established connections between surface area and volume for the study of our question about babies dehydrating?
An approach to this could be:
- What is a better sized rectangular prism for approximating the size of a baby or toddler? Take an age and find out about some possible dimensions to use. Maybe someone in the group has a younger brother or sister they can use as a model?
- What size prism will you use for the adult? Or a teenager?
- Once you agree on a more realistic size for your rectangular prism representations of a baby/toddler and adult/teenager, complete the above analysis using your formulas for comparing their relative surface areas and volume.
- How does this compare with the original analysis using the centimetre cubes?
Or use any other model and the formulas to investigate this question mathematically.
Communicate the results of the problem
So, let’s return to the original question: Why do babies dehydrate faster than adults in summer?
Students should present the findings of their investigation with reference to the original question. They may choose to make a poster that explains why, write a report, or create a multimedia presentation. To support Outcome 3, students should use technology to develop their report and create visualisations of the project and models used.
Maybe students could consider addressing the following questions:
- What would the situation be with regard to cooling, rather than heating? Who would cool the most slowly?
- What factors or issues could also be dependent on this relationship between length, surface area and volume for different objects or items? Buildings, packaging, for example?