Prediction and experiment: change one condition

Where we are on the map

The map shows the reasoning order. Its numbers are worked examples unless the lesson explicitly identifies your own measurement. You are in the first of eleven blocks.

A question from daily life

Two people time swings. The first has a short rope and a strong push; the second has a long rope and a weak push. If the times differ, which difference mattered? We cannot tell from this comparison.

Words and meanings

An experimental condition is something we set, such as rope length. The changed condition is the one difference we want to investigate. Held conditions are things we try to keep the same. A prediction is an expected result written before observation. A hypothesis is a testable proposed explanation; it may be wrong. Agreement with a result does not prove it forever.

Predict before checking

Which takes longer for ten complete swings: a 30 cm or a 60 cm pendulum? Record your answer before seeing any times. It is only a prediction so far.

Do it and record it

Plan on paper: use the same lightweight weight, count ten swings each time, start from a small tilt without pushing, and start the timer in the same way. Change only the distance from the hanging point to the weight’s centre: 30 cm and 60 cm. Actual measurement comes later.

A worked example

Unfair comparison: count 10 swings at 30 cm but 20 at 60 cm. Fair comparison: count 10 at both lengths. Worked values of 11.0 and 15.6 seconds do not establish that their ratio applies under every condition.

A common mistake

‘I changed one condition, so everything else was identical’ is too strong. A person’s reaction when starting the timer and moving air can still vary. That is why we repeat measurements.

A new situation

Two equal damp towels dry, one in shade and one near a window. Which conditions are hard to hold fixed? Do not hang anything on a heater for this exercise.

Notebook entry and self-check

Record your prediction, the one changed condition, three held conditions, and one possible unwanted difference. Was the prediction written before the result?

Next lesson: Quantity, number, and unit