The Formal Report
This section discusses the results of the report. It can also discuss anything of relevance such as the experimental procedure, errors not accounted for by the experimental uncertainties, comparisons to other work (other student results or literature values).
A good discussion might start by summarizing the results of the report (similar to a conclusion) and comparing these results to known values or other student results. By critically analyzing the uncertainties and where they came from one can often gain insight into what experimental values had the greatest influence on the uncertainty in the final result.
Some persons consider a tabulation of "sources of errors" important. This could be presented in the discussion section. However, more important is the consideration of each "source of error" and how it actually influenced your results. The simple tabulation of "sources of errors" is not enough for a physics formal report. For example, in an experiment where temperature and pressure are measured to determine some result, one might find that the temperature measurement contributed most to the final uncertainty. Then if one suspects a systematic error, one can consider how a systematic error in temperature would affect the final result and more importantly, one can estimate by how much.
The discussion will first clearly identify any mistakes made (procedure or analysis) that has impacted the final results. The discussion will discuss the results. The discussion will discuss the uncertainties. If a graph is part of the results, then the discussion will discuss the behaviour of the graph.
The discussion does not contain anything new that belongs in the other report sections. An example for clarification: If a string instead of a clip had been used to attach a weight, the fact that a string was used is reported in the Procedure as this is what you actually used. However the Discussion could state that a clip was not used. The fact that a clip was not used does not belong in the Procedure (because it is not a statement of what was actually done). But it might be a good part of a Discussion if it was a decision that effects the outcome significantly and constitutes a departure from standard practice.
Write in well-constructed paragraphs, a paragraph having a single thought or concept. A shorter paragraph is more easily read and understood than a long one. Do not ramble on. Once written, re-read your discussion and ask yourself if it makes sense. Discussions that are difficult to read or understand are often not read. Also ask yourself if the discussion enhances or depletes your credibility as a scientist. Your credibility is easy to loose, and hard to build, so protect it by being careful of what you say in the discussion section.