Why do this problem?
Learners can discover the base two number system for themselves
and gain experience of working with powers of 2 and 3. The
problem can be used to reinforce understanding of place value.
The generalisation to the problem which allows weights in both
pans leads naturally to representing numbers in base 3.
Possible approach
As a class, work together through weighing 1, 2, 3, 4, 5 and 6
units with the fewest weights possible and then ask the learners
to continue the process.
Key questions Weights in
one pan only
What weights are needed to weigh 1, 2, 3,....15 units?
What pattern do you notice in the weights used? Can you explain
it?
When you include another weight in the set of weights used how
many objects can now be weighed?
How many weights are needed to weigh objects of 1, 2, 3 ... 100
units?
How many weights are needed to weigh 1 to 1000 units or 1 to n
units?
Weights in
both pans
What weights are needed to weigh 1, 2, 3,....13 units?
What pattern do you notice in the weights used? Can you explain
it?
When you include another weight in the set of weights used how
many objects can now be weighed?
How many weights are needed to weigh objects of 1, 2, 3 ... 100
units?
How many weights are needed to weigh 1 to 1000 units or 1 to n
units?
Possible extension
Discuss with the class the idea that, using both pans, the
weights are used either as +1 times the weight (in the opposite
pan to the object), 0 times the weight (not used at all), or -1
times the weight (in the same pan as the object). We are using a
numbering system
a0 + a1x + a2x2 + a3x3 + ...
where the coefficients a0, a1 etc. are all 1, 0 or -1
and x=3. By a (reversible) 'shift' this is changed to the base 3 number
system.
All integers can be written in base 3 with digits 0, 1 and 2, that is as
b0 + b1x + b2x2 + b3x3 + ...
where the coefficients b0, b1 etc. are all 0, 1 or 2
and x=3. This explains why all objects weighing an integer number of units
can be weighed with a set of weights that are all powers of 3.
You may find a spreadsheet
tool useful.