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Science lesson plan · 11th grade (Year 12 in England) · 45 minutes

Conservation of momentum in collisions and explosions

Forces and motion

CCSS NGSS.HS-PS2-2England: GCSE Combined Science: Forces (conservation of momentum, Higher)

Objective and success criteria

Students will: Conservation of momentum in collisions and explosions.

  • I can answer questions on today's skill on my own, without help.
  • I can explain each step and say why it works.
  • I can check my answer and spot a mistake.

Before and after

Needs first: Momentum (p = m v)

Leads to: the next skill in this area.

Materials: Force meters, Toy cars and ramps, Mini whiteboards, A diagram or short video clip of the idea, Printed student sheet (below).

Warm-up · 5 min

  • 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 2 kg object moving at 25 m/s, in kg m/s? → 50 kgm/sPrompt: momentum = mass × velocity
  2. An object moving at 3 m/s has a momentum of 6 kg m/s. What is its mass, in kg? → 2 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? (they have the same momentum / the ball / neither has any / the runner) → they have the same momentumPrompt: Work out m × v for each.

Hook · 5 min

  • 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.

I do: model it · 10 min

  • 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.

We do: guided practice · 10 min

  • 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 1 kg trolley moving at 3 m/s hits a stationary 2 kg trolley. They stick together. What is their velocity just after the collision, in m/s? (Give 2 decimal places.) → 1 m/sPrompt: Total momentum before = total momentum after.
  2. A 5 kg toy cannon at rest fires a 0.05 kg ball forwards at 400 m/s. What is the recoil speed of the cannon, in m/s? → 4 m/sPrompt: The total momentum before firing is zero.

You do: independent practice · 10 min

  • 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.

Exit ticket · 5 min

  • Two questions on their own. Sort answers into got it / nearly / not yet to plan the next lesson.
  1. A 4 kg toy cannon at rest fires a 0.05 kg ball forwards at 200 m/s. What is the recoil speed of the cannon, in m/s? → 2.5 m/sPrompt: The total momentum before firing is zero.
  2. A 1200 kg car moving at 4 m/s hits a stationary 4000 kg car. They lock together. What is their velocity just after the collision, in m/s? (Give 2 decimal places.) → 0.92 m/sPrompt: Total momentum before = total momentum after.

Watch out for

  • Everyday meanings of words getting in the way of the science meaning: define key words explicitly.
  • Remembering facts without the reason: ask "why?" and "what is the evidence?"

Support and stretch

Support

  • Review first: Momentum (p = m v).
  • Use concrete materials: force meters.
  • Give the first step of each question, or a partly worked example to finish.
  • Pair with a confident partner for the guided practice.

Stretch

  • Explain a common mistake in writing and how to fix it.
  • Write a word problem that uses today's skill, then solve a partner's.

Homework and more practice

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

Free lesson plan from eurekami.com/lesson-plan-builder

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Conservation of momentum in collisions and explosions

Name

Date

Score/ 8

  1. Question 1

    In a car crash test, momentum is conserved but kinetic energy is not. Where does the kinetic energy go?

    • A. into the cars' nuclear store
    • B. into deforming the cars, heating them and sound
    • C. it is destroyed
    • D. into extra momentum

  2. Question 2

    A 4 kg toy cannon at rest fires a 0.05 kg ball forwards at 300 m/s. What is the recoil speed of the cannon, in m/s?

    Answerm/s

  3. Question 3

    A 3 kg trolley moving at 6 m/s hits a stationary 2 kg trolley. They stick together. What is their velocity just after the collision, in m/s? (Give 2 decimal places.)

    Answerm/s

  4. Question 4

    A 2 kg toy cannon at rest fires a 0.01 kg ball forwards at 400 m/s. What is the recoil speed of the cannon, in m/s?

    Answerm/s

  5. Question 5

    A 1000 kg car moving at 6 m/s hits a stationary 800 kg car. They lock together. What is their velocity just after the collision, in m/s? (Give 2 decimal places.)

    Answerm/s

  6. Question 6

    Why does a rocket in space accelerate forwards when it pushes exhaust gases backwards?

    • A. Gravity pushes it
    • B. Momentum is not conserved in space
    • C. The gases gain backward momentum, so the rocket gains equal forward momentum
    • D. It pushes against the air

  7. Question 7

    A 2 kg toy cannon at rest fires a 0.01 kg ball forwards at 200 m/s. What is the recoil speed of the cannon, in m/s?

    Answerm/s

  8. Question 8

    A 1000 kg car moving at 6 m/s hits a stationary 1000 kg car. They lock together. What is their velocity just after the collision, in m/s? (Give 2 decimal places.)

    Answerm/s

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Answer key: Conservation of momentum in collisions and explosions

  1. 1. B. into deforming the cars, heating them and sound

  2. 2. 3.75 m/s

  3. 3. 3.6 m/s

  4. 4. 2 m/s

  5. 5. 3.33 m/s

  6. 6. C. The gases gain backward momentum, so the rocket gains equal forward momentum

  7. 7. 1 m/s

  8. 8. 3 m/s