• Quick retrieval from earlier learning (Repeat readings, means and anomalies; Converting units). Students answer on mini whiteboards.
• Show answers together and note who needs support in the main task.

1. A group measured the time for a ball to roll down a ramp three times: 30, 28 and 28 s. What is the mean? Give your answer to 1 decimal place. → 28.7 sPrompt: Add the three readings.
2. Convert 3 km into m. → 3000 mPrompt: How many m are in 1 km?
3. Four repeat readings of the temperature rise in a reaction (°C): 19, 29, 18, 18. Which reading is anomalous (does not fit the pattern)? (29 / 18 / 19) → 29Prompt: Most of the readings are close together.

• Show a simple experiment with one thing changed and ask what makes it a fair test.
• Share the objective: Accuracy, precision, errors and significant figures. Read the success criteria together.

• Work through the example out loud, saying why you do each step, not just what you do.
• Example: A balance shows 0.3 g when nothing is on it. What kind of error is this?
• Step 1: A scale that does not start at zero adds the same error every time: a systematic (zero) error.
• Step 2: So the answer is: a zero error (a systematic error)
• Answer: a zero error (a systematic error)
• Pause after each step and ask a student to say what comes next.

• Do these together. Students try each step on whiteboards before you reveal it.
• Use the hints as prompts rather than giving the answer.

1. Readings of the same length: 12.1, 12.1, 12.2, 12.1 cm. The true length is 13.0 cm. The readings are… (neither repeatable nor close / accurate but not precise / accurate and precise / precise but not accurate) → precise but not accuratePrompt: Precise means close to each other. Accurate means close to the true value.
2. A balance reads 0.5 g with nothing on it. With a rock on it, it reads 10.2 g. What is the true mass of the rock, in g? → 9.7 gPrompt: The zero error is included in every reading.

• Students work on the student sheet. Circulate and check the first two answers of every student early.
• Students who finish: write a question of their own for a partner, with the answer.

The 8 questions are on the student sheet below, with answers in the key.

• Two questions on their own. Sort answers into got it / nearly / not yet to plan the next lesson.

1. Timing a falling ball by hand gives slightly different values each time. This is mainly due to… (a zero error / gravity switching off / the ball changing mass / random error (human reaction time)) → random error (human reaction time)Prompt: The errors are different each time, not the same.
2. A calculator gives a density of 0.023345. Write it to 2 significant figures. → 0.023Prompt: Count from the first non-zero digit.

Free online practice with hints and printable worksheets for this skill: eurekami.com/science/grade-9.