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Big and Small Numbers in Physics - Group Task

Stage: 5 Challenge Level: Challenge Level:1

The notes below describe a method of engagement based around a technique called complex instruction. You can see many examples of tasks which work well in groups on the May 2010 NRICH publication.

Although this problem is group-worthy, it can, of course, be attempted individually if you wish.

Why do this problem?


The problem involves working out sensible approximations to physical quantities. Making good headway into the task will be difficult and requires good use of teamwork.


Possible approach

This is an ideal problem for students to tackle in groups of four. Allocating these clear roles (Word, pdf) can help the group to work in a purposeful way - success on this task should be measured by how effectively the members of the group work together as well as by the solutions they reach.



You may want to make calculators, internet, squared or graph paper, poster paper, and coloured pens available for the Resource Manager in each group to collect. As teacher, you (or the internet) will be a resource containing knowledge of physical data, constants and formulae.

While groups are working divide the board up with the groups names as headings. Listen in on what groups are saying, and use the board to jot down comments and feedback to the students about the way they are working together. This is a good way of highlighting the mathematical behaviours you want to promote, particularly with a challenging task such as this.


Group A - Good to see you sharing different ways of thinking about the problem.
Group B - I like the way you are keeping a record of people's ideas and results.
Group C - Resource manager - is there anything your team needs? Are there any facts or data that you need but don't know?

Alternatively, your focus for feedback might be mathematical:

Group A - I like the way you set out your assumptions clearly.
Group B - Can you provide some error bounds on that calculation?
Group C - Good to see that someone's checking the numerical calculations.

Make sure that while groups are working they are reminded of the need to be ready to present some of their approximations at the end, and that all are aware of how long they have left.

We assume that each group will record their reasoning, assumptions and calculations in preparation for reporting back. There are many ways that groups can report back. Here are just a few suggestions:
  • Every group is given a couple of minutes to report back to the whole class, perhaps focussing on explaining two of their approximations. Those listening can seek clarification and ask questions. After each presentation, those listening are invited to offer positive feedback. Finally, those presenting can suggest how the group could have improved their work on the task.
  • Two people from each group move to join an adjacent group. The two "hosts" explain their some of their calculations and reasoning to the two "visitors". The "visitors" act as critical friends, requiring clear mathematical explanations and justifications. The "visitors" then comment on anything they did differently in their own group.

Key questions

If your focus is effective group work, this list of skills may be helpful (Word, PDF). Ask learners to identify which skills they demonstrated, and which skills they need to develop further.

If your focus is mathematical, these prompts might be useful:
What assumptions have you made?
What other information do you need?
Are there any questions which give an exact answer?
Can you say anything about the accuracy of those answers which aren't exact?

Possible extension

Although this is an approximations question, it is possible to give rigorous bounds on the quantities. Students could be encouraged to give a known upper and lower bound for each quantity, with the focus on providing as tight a bound as possible

Students could also compare their approximations with those of others. Is there a sense in which one of best?

Possible support

By working in groups with clearly assigned roles we are encouraging students to take responsibility for ensuring that everyone understands before the group moves on.