• 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 70 kg object moving at 10 m/s, in kg m/s? → 700 kgm/sPrompt: momentum = mass × velocity
2. An object moving at 6 m/s has a momentum of 480 kg m/s. What is its mass, in kg? → 80 kgPrompt: Rearrange p = m v.
3. Why is momentum a vector quantity? (It depends on velocity, which has a direction / It is always zero / It is measured in joules / It depends only on mass) → It depends on velocity, which has a directionPrompt: p = m × v

• Drop a sheet of paper and a scrunched-up sheet together and ask why one falls faster.
• Share the objective: Conservation of momentum in collisions and explosions. Read the success criteria together.

• Work through the example out loud, saying why you do each step, not just what you do.
• Example: A 2 kg trolley moving at 4 m/s hits a stationary 4 kg trolley. They stick together. What is their velocity just after the collision, in m/s? (Give 2 decimal places.)
• Step 1: 2 × 4 = 8
• Step 2: v = 8 ÷ 6 = 1.333
• Step 3: So the answer is: 1.33 m/s
• Answer: 1.33 m/s
• 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 2 kg trolley moving at 4 m/s hits a stationary 1 kg trolley. They stick together. What is their velocity just after the collision, in m/s? (Give 2 decimal places.) → 2.67 m/sPrompt: Total momentum before = total momentum after.
2. A 4 kg toy cannon at rest fires a 0.02 kg ball forwards at 400 m/s. What is the recoil speed of the cannon, in m/s? → 2 m/sPrompt: The total momentum before firing is zero.

• 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 1 kg trolley moving at 12 m/s hits a stationary 4 kg trolley. They stick together. What is their velocity just after the collision, in m/s? (Give 2 decimal places.) → 2.4 m/sPrompt: Total momentum before = total momentum after.
2. Two ice skaters at rest push apart. Skater A (40 kg) moves off at 3 m/s. What is the total momentum of the two skaters afterwards? (0 kg m/s / 120 kg m/s / 240 kg m/s / 40 kg m/s) → 0 kg m/sPrompt: What was it before they pushed?

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