Let the numbers decide

Two explanations can both sound plausible. Only a calculation tells you which one the evidence actually supports.

Due: 28 September 2026 · Weight: 30%

Week 7 is where this course stopped settling questions by arguing and started settling them by calculating. Chemical burning sounded plausible for the Sun, right up until the arithmetic gave it a lifetime of thousands of years instead of billions. Gravity sounded better, and still fell short by three orders of magnitude. Nothing about either explanation looked obviously wrong on its face. The numbers did the work that argument could not.

The brief#

Choose two plausible explanations for the same physical situation, and use estimates or calculation to decide between them.

Find a different situation where scale is the thing that distinguishes a good explanation from a bad one — not the Sun’s lifetime itself, which Week 7 already used, but a comparably decisive case. Set out what each explanation predicts, put a number on it, and say what that number actually rules out or fails to rule out. An estimate is enough; you do not need exact figures, only figures accurate enough to bear the weight of the conclusion.

The point of the exercise is not arithmetic for its own sake. If the calculation does not change what you believe, or could not have changed it no matter what the numbers came out to, you have not found the right comparison yet. Choose a case where the outcome of the sum is genuinely in question before you do it.

What you submit#

A short written piece, around 800 to 1,200 words, that states both explanations, shows your working, and ends with a judgement the calculation earns. Show the estimates you used and where they came from — a number pulled from nowhere is not a calculation.