• 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 how far a toy car travelled three times: 85, 84 and 84 cm. What is the mean? Give your answer to 1 decimal place. → 84.3 cmPrompt: Add the three readings.
2. Convert 6 cm into mm. → 60 mmPrompt: How many mm are in 1 cm?
3. Four repeat readings of a pulse rate after exercise (bpm): 58, 108, 110, 107. Which reading is anomalous (does not fit the pattern)? (58 / 108 / 107 / 110) → 58Prompt: 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. A balance shows 0.3 g when nothing is on it. What kind of error is this? (no error / an anomalous result / a zero error (a systematic error) / a random error) → a zero error (a systematic error)Prompt: It shifts every reading by the same amount.
2. A balance reads −0.2 g with nothing on it. With a rock on it, it reads 36.4 g. What is the true mass of the rock, in g? → 36.6 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 57.7 g. What is the true mass of the rock, in g? → 57.4 gPrompt: The zero error is included in every reading.
2. Timing a falling ball by hand gives slightly different values each time. This is mainly due to… (random error (human reaction time) / the ball changing mass / a zero error / gravity switching off) → random error (human reaction time)Prompt: The errors are different each time, not the same.

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