• 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: 35, 36 and 37 s. What is the mean? Give your answer to 1 decimal place. → 36 sPrompt: Add the three readings.
2. Convert 3 kg into g. → 3000 gPrompt: How many g are in 1 kg?
3. Four repeat readings of a pulse rate after exercise (bpm): 200, 131, 133, 134. Which reading is anomalous (does not fit the pattern)? (200 / 131 / 133 / 134) → 200Prompt: 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. Timing a falling ball by hand gives slightly different values each time. This is mainly due to… (random error (human reaction time) / a zero error / the ball changing mass / gravity switching off) → random error (human reaction time)Prompt: The errors are different each time, not the same.
2. A balance reads 0.3 g with nothing on it. With a rock on it, it reads 59.2 g. What is the true mass of the rock, in g? → 58.9 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 calculator gives a density of 0.09115. Write it to 2 significant figures. → 0.091Prompt: Count from the first non-zero digit.
2. What is the resolution of a ruler marked in millimetres? (1 cm / 1 mm / 0.1 mm / 1 m) → 1 mmPrompt: Resolution is the smallest change the instrument can show.

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