Lesson 2 Two variables
This is a ‘circus’ of small investigations each of which involves two variables. Activity 1 is a simple pulley where mass and force are measured (as the input goes up, so does the outcome). Activity 2 involves pouring the same amount of water into several different containers of different widths (as the input variable, width, gets bigger, the outcome variable, depth, gets smaller or vice versa).
Activity 3 looks at which surface of drying leaves loses the most water by preventing water loss from different surfaces with Vaseline. There is no easy input/outcome relationship here. It cannot be quantified, only described. The fourth activity is a set of data showing children of different heights and weights and there is no relationship between the variables. Some schools have been concerned about drawing attention to the weight issue so do substitute a different set of data if you want to. Two or more different tree species of different heights or widths and different ages for example.
There are two main purposes to the lesson. First to introduce the terms ‘input’ and ‘outcome’ variables, which we suggest as alternatives to the more usual, but confusing terms ‘independent’ and ‘dependent’. At a later stage, probably from Y9 or S3 onwards
you will find you can easily substitute input and outcome for independent and dependent variables. Second, to consolidate notions of variables and relationships from Lesson 1. This lesson gives pupils opportunities to express different kinds of relationships in their own words. The lesson operates mainly at a concrete level with very little opportunity for higher level cognitive conflict. The lack of relationship between height and weight in the fourth activity gives some pause for thought but is easily solvable using a concrete strategy of ‘ranking’. The third investigation with leaves requires a different kind of articulation. There is a deductive reasoning chain something like, ‘if the water gets out of this surface more than that surface, then when this surface is covered it will stop that happening, therefore I would expect the leaf to stay fresh’. This long chain is demanding but it is worthwhile trying to get pupils to think, ‘if this …
then … that’. Do not worry if all groups have not covered all activities. The whole-class discussion will compensate for this. Remember good ideas are contagious and a child does not have to think it out first for himself to benefit from hearing peers work through the problem.
New key words: input variable, outcome variable, data