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Science lesson plan · 8th grade (Year 9 in England) · 45 minutes

Moments and levers

Forces and motion

CCSS NGSS.MS-PS2-2England: KS3 Physics: Motion and forces (balanced forces)England: KS2 Y5 Forces

Objective and success criteria

Students will: Moments and levers.

  • 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: Mass and weight (W = mg)

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 (Mass and weight (W = mg)). Students answer on mini whiteboards.
  • Show answers together and note who needs support in the main task.
  1. What is the weight of a 75 kg object on Earth? Use g = 9.8 N/kg. → 735 NPrompt: weight = mass × gravitational field strength (W = m g)
  2. An astronaut's mass is 50 kg. Gravitational field strength on the Moon is 1.6 N/kg. What would their weight be there, in N? → 80 NPrompt: Mass stays the same everywhere; weight changes.
  3. Mass and weight: which statement is NOT true? (You would weigh less on the Moon than on Earth. / Weight is measured in kilograms. / Mass is measured in kilograms. / Your mass is the same on the Moon as on Earth.) → Weight is measured in kilograms.Prompt: Which one changes on the Moon?

Hook · 5 min

  • Drop a sheet of paper and a scrunched-up sheet together and ask why one falls faster.
  • Share the objective: Moments and levers. 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 force of 5 N is applied 1.5 m from a pivot, at right angles. What is the moment, in N m?
  • Step 1: 5 × 1.5 = 7.5
  • Step 2: So the answer is: 7.5 N m
  • Answer: 7.5 Nm
  • 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 force of 50 N is applied 0.25 m from a pivot, at right angles. What is the moment, in N m? → 12.5 NmPrompt: moment = force × perpendicular distance from the pivot
  2. On a seesaw, a child weighing 300 N sits 1 m from the pivot. How heavy must a second child sitting 2.5 m on the other side be to balance it, in N? → 120 NPrompt: Balanced means clockwise moment = anticlockwise moment.

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. On a seesaw, a child weighing 300 N sits 1 m from the pivot. How heavy must a second child sitting 2.5 m on the other side be to balance it, in N? → 120 NPrompt: Balanced means clockwise moment = anticlockwise moment.
  2. A force of 10 N is applied 0.2 m from a pivot, at right angles. What is the moment, in N m? → 2 NmPrompt: moment = force × perpendicular distance from the pivot

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: Mass and weight (W = mg).
  • 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-8.

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

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Moments and levers

Name

Date

Score/ 8

  1. Question 1

    A force of 50 N is applied 0.5 m from a pivot, at right angles. What is the moment, in N m?

    AnswerNm

  2. Question 2

    On a seesaw, a child weighing 400 N sits 2 m from the pivot. How heavy must a second child sitting 3 m on the other side be to balance it, in N?

    AnswerN

  3. Question 3

    Why is it easier to open a door by pushing far from the hinges?

    • A. Hinges push back
    • B. The door is lighter there
    • C. A bigger distance from the pivot gives a bigger moment for the same force
    • D. There is less friction there

  4. Question 4

    A force of 10 N is applied 0.5 m from a pivot, at right angles. What is the moment, in N m?

    AnswerNm

  5. Question 5

    On a seesaw, a child weighing 300 N sits 2 m from the pivot. How heavy must a second child sitting 2.5 m on the other side be to balance it, in N?

    AnswerN

  6. Question 6

    Why does a long spanner (wrench) make it easier to undo a tight nut?

    • A. The longer handle increases the moment
    • B. It removes friction
    • C. It reduces the mass of the nut
    • D. It is made of a stronger metal

  7. Question 7

    A force of 20 N is applied 0.2 m from a pivot, at right angles. What is the moment, in N m?

    AnswerNm

  8. Question 8

    On a seesaw, a child weighing 200 N sits 1 m from the pivot. How heavy must a second child sitting 1.5 m on the other side be to balance it, in N?

    AnswerN

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Answer key: Moments and levers

  1. 1. 25 Nm

  2. 2. 266.67 N

  3. 3. C. A bigger distance from the pivot gives a bigger moment for the same force

  4. 4. 5 Nm

  5. 5. 240 N

  6. 6. A. The longer handle increases the moment

  7. 7. 4 Nm

  8. 8. 133.33 N