• 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: 32, 35 and 31 s. What is the mean? Give your answer to 1 decimal place. → 32.7 sPrompt: Add the three readings.
2. Convert 3 min into s. → 180 sPrompt: How many s are in 1 min?
3. Four repeat readings of how far a toy car travelled (cm): 88, 89, 51, 90. Which reading is anomalous (does not fit the pattern)? (51 / 90 / 89 / 88) → 51Prompt: 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… (precise but not accurate / accurate but not precise / accurate and precise / neither repeatable nor close) → precise but not accuratePrompt: Precise means close to each other. Accurate means close to the true value.
2. A balance reads 0.2 g with nothing on it. With a rock on it, it reads 27.5 g. What is the true mass of the rock, in g? → 27.3 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. A balance reads −0.3 g with nothing on it. With a rock on it, it reads 44.2 g. What is the true mass of the rock, in g? → 44.5 gPrompt: The zero error is included in every reading.
2. A calculator gives a density of 98.466. Write it to 2 significant figures. → 98Prompt: Count from the first non-zero digit.

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