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Science lesson plan · 7th grade (Year 8 in England) · 30 minutes

Stretching springs: extension and Hooke's law

Forces and motion

CCSS NGSS.MS-PS2-2CCSS NGSS.HS-PS2-1England: KS3 Physics: Motion and forces (forces and elasticity)England: GCSE Combined Science: Forces (required practical: force and extension)

Objective and success criteria

Students will: Stretching springs: extension and Hooke's law.

  • 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: Energy stored in a stretched spring (E = ½ k e²)

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

Warm-up · 4 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 50 kg object on Earth? Use g = 9.8 N/kg. → 490 NPrompt: weight = mass × gravitational field strength (W = m g)
  2. An astronaut's mass is 10 kg. Gravitational field strength on Mars is 3.7 N/kg. What would their weight be there, in N? → 37 NPrompt: Mass stays the same everywhere; weight changes.
  3. Mass and weight: which statement is NOT true? (Weight is a force measured in newtons. / Your mass is smaller on the Moon. / Mass is measured in kilograms. / You would weigh less on the Moon than on Earth.) → Your mass is smaller on the Moon.Prompt: Which one changes on the Moon?

Hook · 3 min

  • Drop a sheet of paper and a scrunched-up sheet together and ask why one falls faster.
  • Share the objective: Stretching springs: extension and Hooke's law. Read the success criteria together.

I do: model it · 7 min

  • Work through the example out loud, saying why you do each step, not just what you do.
  • Example: A spring is 15 cm long. When a weight is hung on it, it is 17 cm long. What is the extension, in cm?
  • Step 1: 17 − 15 = 2
  • Step 2: So the answer is: 2 cm
  • Answer: 2 cm
  • Pause after each step and ask a student to say what comes next.

We do: guided practice · 6 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 spring is 25 cm long. When a weight is hung on it, it is 27 cm long. What is the extension, in cm? → 2 cmPrompt: Extension is how much longer the spring has become.
  2. A spring has a spring constant of 20 N/m. What force is needed to stretch it by 5 cm, in N? (F = k × e, with e in metres) → 1 NPrompt: Convert the extension to metres first.

You do: independent practice · 7 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 6 questions are on the student sheet below, with answers in the key.

Exit ticket · 3 min

  • Two questions on their own. Sort answers into got it / nearly / not yet to plan the next lesson.
  1. What does it mean if a spring goes past its elastic limit? (It snaps back faster / It will not return to its original length when the force is removed / Its spring constant doubles / It becomes stronger) → It will not return to its original length when the force is removedPrompt: Think about an over-stretched hair tie.
  2. A force of 0.8 N stretches a spring by 2 cm. What is the spring constant, in N/m? → 40 N/mPrompt: k = F ÷ e

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

  • Move on to: Energy stored in a stretched spring (E = ½ k e²).
  • 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-7.

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

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Stretching springs: extension and Hooke's law

Name

Date

Score/ 6

  1. Question 1

    A spring has a spring constant of 50 N/m. What force is needed to stretch it by 5 cm, in N? (F = k × e, with e in metres)

    AnswerN

  2. Question 2

    A spring is 25 cm long. When a weight is hung on it, it is 29 cm long. What is the extension, in cm?

    Answercm

  3. Question 3

    A graph of force against extension is a straight line through the origin. What does this show?

    • A. Extension does not depend on force
    • B. The spring is broken
    • C. The spring has passed its elastic limit
    • D. Force and extension are directly proportional

  4. Question 4

    A spring has a spring constant of 80 N/m. What force is needed to stretch it by 2 cm, in N? (F = k × e, with e in metres)

    AnswerN

  5. Question 5

    A spring is 25 cm long. When a weight is hung on it, it is 30 cm long. What is the extension, in cm?

    Answercm

  6. Question 6

    Two springs are stretched by the same force. Spring A stretches less than B. Which has the larger spring constant?

    • A. spring B
    • B. spring A (it is stiffer)
    • C. it cannot be known
    • D. they are equal

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Answer key: Stretching springs: extension and Hooke's law

  1. 1. 2.5 N

  2. 2. 4 cm

  3. 3. D. Force and extension are directly proportional

  4. 4. 1.6 N

  5. 5. 5 cm

  6. 6. B. spring A (it is stiffer)