An estimate is not a guess. It is the result of a rational procedure.

Learning objective Make models using imagination and three techniques:

• Make oversimplifying assumptions
• Break big problems into smaller problems
• Make estimates or even guesses to solve the smaller problems.

Some tips

• Ask: “What information would help me solve the problem?”
• Use rough data from readily available sources
• When in doubt, guess to nearest power of ten (logarithmic scale). . If one power of ten seems too low and the next seems too high, try their geometric mean.

Before class

• Watch Powers of Ten
• Find out who Enrico Fermi was
• Read about the famous problem of the number of piano tuners in Chicago
• See how a Fermi problem can be tackled.
• Peruse Frontiers of Science Chapter 2 (actually the whole book is interesting)
• Explore screencast: Practical advice on solving Fermi problems, and some examples in the second half
• Consult the main points of the lesson

In class

• Solve some Fermi problems, working in groups