Use a ruler to compare moments around a pivot

Support the ruler near its balance point. Put a small known load on one side and note the distance from the support to the middle of the load.

Balance a ruler before placing a pair of unequal clean weights across its support. If the results vary, look for a second feature that moved along with the one you meant to change. That can be the distance to an observer, the angle of a support or the way the material moves between runs. Isolate it in a small follow-up trial when possible.

A smaller weight placed farther away can balance a heavier one nearer the pivot. Observe the setup from the same place in each trial. A view from farther away can hide a shallow curve or make two positions look aligned. A short guide mark shows the place you used. Keep the recording method fixed as well when changing the setup itself is the point of the comparison.

Adjust an unequal load on the other side until the ruler rests nearly horizontal. Compare load multiplied by distance on each side.

A broad resting surface can hold several nearly level positions without revealing the exact balance point. A useful prediction can say which direction an effect should move without specifying its exact size. Put that prediction next to the expected observation time or distance. If the pattern appears in the other direction, note how you would test whether it arose from a measurement condition or from your original explanation.

Try a narrower support or read the point where the ruler just begins to tip. Make a simple model of what you believe happens between the starting and final positions. Then check one point at which that model gives a visible consequence. A single carefully observed intermediate position can be more revealing than comparing two widely separated endpoints with an unknown change between them.

Record distances to the centres of the weights and compare the products. Sketch your apparatus before you start and use the sketch to identify where each observation will be made. Write down the starting state before changing it. If another person repeats the trial, knowing the relative positions can matter more than the brand or appearance of the materials.

Write a row for each run and keep apparently odd results in the record. Mark things that went wrong, such as a shifted surface, uneven starting point or missed time, before judging the pattern. Changing how you record the behaviour halfway through can make two otherwise similar trials difficult to compare. Explain the outcome in a sentence that a reader can check against the figures or sketch. Use a second sentence for a proposed cause. If you can imagine a different cause that fits the same observations, describe a small test that would help distinguish the two. Comparing those predictions can make a follow-up clearer than simply repeating the same demonstration. Record whether the ruler is balanced with its own centre above the support. A small unbalanced portion contributes its own turning effect. Write the independent change at the start of each row and keep the outcome beside it. Include anything about a failed run that would matter to somebody repeating the setup. If you later make a neat graph, keep the rough record as well so it remains possible to inspect the reasons for leaving out a point.

Repeat with a new pair and note the effect of friction or a broad support on your ability to locate balance. Write one prediction alongside the setup. It helps to describe what would be observed if that prediction were right, including a direction, a clearer pattern or a measurable difference. This makes room for the outcome to disagree with what you expected and gives you a better question for a second run.