What does a pan balance tell us about the weight order of the cubes?
3. Analyze the data
Ask students to identify instances where information (data) from the pan balance was different from the order they found using felt weight. Mark these on the class data tables.
- When you compare the two data tables, can you find any differences in the order of the cubes by weight? Where are the differences?
Make claims based on evidence
Encourage students to identify patterns in the data — places of agreement and places of differences — and to account for the patterns with some sort of hypothesis. They will likely find that the two orderings differ most in the areas that the pan balance showed the weights to be most similar. Students might propose, correctly, that small differences in weight are difficult to distinguish by hand.
Note: The ability of our hands to sense weight differences depends not only on the relative weights of the objects but also on their actual weights. For example, our hands can tell the difference between objects weighing 20 grams and 40 grams, but probably not between objects weighing 0.2 grams and 0.4 grams, even though in both cases the weights differ by a factor of 2.
Students will find that their hands are not consistently reliable for ordering objects by weight, especially when weight differences are small. The pan balance provides more accurate and consistent results, but instruments also have their limits. Inexpensive pan balances can lose effectiveness once the difference in weight between two objects drops much below 1 gram.
