How do I run a crater experiment as a fair test?
Change one variable, such as drop height or ball mass, keep everything else the same, repeat each drop three times and measure crater diameter carefully.
The flour-tray crater experiment is ideal for teaching scientific method, because pupils can see results immediately and the variables are easy to control.
Set up. Fill a deep tray with about 5 centimetres of plain flour, level it with a ruler, and dust the top with cocoa powder so ejecta shows clearly. Have a set of balls or marbles, a metre rule, a ruler or callipers for measuring, and a results table. Cover the floor with newspaper.
Choose a question. For example: How does drop height affect crater size? Or: Does a heavier ball make a bigger crater?
Control variables. If testing height, keep the same ball, the same flour depth and surface, and drop vertically each time. If testing mass, use balls of the same size but different materials, such as a wooden bead, a glass marble and a steel ball bearing.
Measure. Measure crater diameter across the rim, in two directions at right angles, and average them. Optionally measure depth with a cocktail stick and the length of the longest ray. Repeat each trial three times and smooth the surface between drops.
Analyse. Pupils plot diameter against height or mass, describe the pattern and suggest explanations involving energy. Older pupils can calculate gravitational potential energy and compare it with crater size.
Evaluate. Discuss errors and how real impacts differ: real impactors are far faster, so most of the impactor melts or vaporises.