• Quick retrieval from earlier learning (Momentum (p = m v)). Students answer on mini whiteboards.
• Show answers together and note who needs support in the main task.

1. What is the momentum of a 1200 kg object moving at 4 m/s, in kg m/s? → 4800 kgm/sPrompt: momentum = mass × velocity
2. An object moving at 4 m/s has a momentum of 200 kg m/s. What is its mass, in kg? → 50 kgPrompt: Rearrange p = m v.
3. A 0.15 kg ball moving at 20 m/s and a 60 kg runner moving at 0.05 m/s — which has more momentum? (the runner / neither has any / the ball / they have the same momentum) → they have the same momentumPrompt: Work out m × v for each.

• Drop a sheet of paper and a scrunched-up sheet together and ask why one falls faster.
• Share the objective: Force and rate of change of momentum (safety design). Read the success criteria together.

• Work through the example out loud, saying why you do each step, not just what you do.
• Example: A 80 kg passenger moving at 15 m/s is brought to rest in 0.4 s by a seat belt and airbag. What is the average force on the passenger, in N?
• Step 1: Δp = 80 × 15 = 1200 kg m/s
• Step 2: F = 1200 ÷ 0.4 = 3000
• Step 3: So the answer is: 3000 N
• Answer: 3000 N
• 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 70 kg passenger moving at 10 m/s is brought to rest in 0.5 s by a seat belt and airbag. What is the average force on the passenger, in N? → 1400 NPrompt: F = change in momentum ÷ time
2. A helmet increases the time for a head to stop in a fall from 0.05 s to 0.25 s. The change in momentum is the same. By what factor is the average force reduced? → 5Prompt: F = Δp ÷ t, so force is inversely proportional to time.

• 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. Why do cricket or baseball fielders pull their hands back as they catch a fast ball? (It increases the time to stop the ball, reducing the force on their hands / It makes the ball lighter / It increases the ball's speed / It changes the ball's momentum to zero instantly) → It increases the time to stop the ball, reducing the force on their handsPrompt: The change in momentum is the same either way.
2. A helmet increases the time for a head to stop in a fall from 0.05 s to 0.2 s. The change in momentum is the same. By what factor is the average force reduced? → 4Prompt: F = Δp ÷ t, so force is inversely proportional to time.

Free online practice with hints and printable worksheets for this skill: eurekami.com/science/grade-11.