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Science lesson plan · 7th grade (Year 8 in England) · 60 minutes

Mass and weight (W = mg)

Forces and motion

CCSS NGSS.MS-PS2-4CCSS NGSS.HS-PS2-4England: KS3 Physics: Motion and forces (forces)England: KS4 Combined Science: Forces

Objective and success criteria

Students will: Mass and weight (W = mg).

  • 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: Gravity

Leads to: Moments and levers; Force, mass and acceleration (F = ma); Kinetic and gravitational potential energy

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

Warm-up · 6 min

  • Quick retrieval from earlier learning (Gravity). Students answer on mini whiteboards.
  • Show answers together and note who needs support in the main task.
  1. On the Moon, an astronaut dropped a hammer and a feather at the same time. What happened? (The feather landed first / The hammer landed first / They hit the ground together / They floated away) → They hit the ground togetherPrompt: The Moon has no air.
  2. Gravity: which statement is true? (The Earth's gravity keeps the Moon in orbit. / Heavy objects always fall faster than light ones. / There is no gravity in space. / Gravity only acts on things that are falling.) → The Earth's gravity keeps the Moon in orbit.Prompt: What keeps the Moon going round the Earth?
  3. Gravity: which statement is NOT true? (Without air resistance, a heavy and a light object fall at the same rate. / Gravity is weaker on the Moon than on Earth. / There is no gravity in space. / Gravity is a force that attracts objects with mass towards each other.) → There is no gravity in space.Prompt: Why does the space station stay in orbit?

Hook · 6 min

  • Drop a sheet of paper and a scrunched-up sheet together and ask why one falls faster.
  • Share the objective: Mass and weight (W = mg). Read the success criteria together.

I do: model it · 12 min

  • Work through the example out loud, saying why you do each step, not just what you do.
  • Example: What is the weight of a 75 kg object on Earth? Use g = 9.8 N/kg.
  • Step 1: W = 75 × 9.8 = 735
  • Step 2: So the answer is: 735 N
  • Answer: 735 N
  • Pause after each step and ask a student to say what comes next.

We do: guided practice · 12 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. What is the weight of a 8 kg object on Earth? Use g = 9.8 N/kg. → 78.4 NPrompt: weight = mass × gravitational field strength (W = m g)
  2. An astronaut's mass is 20 kg. Gravitational field strength on Mars is 3.7 N/kg. What would their weight be there, in N? → 74 NPrompt: Mass stays the same everywhere; weight changes.

You do: independent practice · 17 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 · 7 min

  • Two questions on their own. Sort answers into got it / nearly / not yet to plan the next lesson.
  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. Mass and weight: which statement is NOT true? (Mass and weight are the same thing. / Weight is a force measured in newtons. / You would weigh less on the Moon than on Earth. / Mass is measured in kilograms.) → Mass and weight are the same thing.Prompt: Which one changes on the Moon?

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: Gravity.
  • 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: Moments and levers; Force, mass and acceleration (F = ma); Kinetic and gravitational potential energy.
  • 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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Mass and weight (W = mg)

Name

Date

Score/ 8

  1. Question 1

    What is the weight of a 75 kg object on Earth? Use g = 9.8 N/kg.

    AnswerN

  2. Question 2

    An astronaut's mass is 10 kg. Gravitational field strength on Jupiter is 25 N/kg. What would their weight be there, in N?

    AnswerN

  3. Question 3

    Mass and weight: which statement is NOT true?

    • A. Mass and weight are the same thing.
    • B. Weight is a force measured in newtons.
    • C. Mass is measured in kilograms.
    • D. Your mass is the same on the Moon as on Earth.
  4. Question 4

    An astronaut's mass is 40 kg. Gravitational field strength on the Moon is 1.6 N/kg. What would their weight be there, in N?

    AnswerN

  5. Question 5

    What is the weight of a 5 kg object on Earth? Use g = 9.8 N/kg.

    AnswerN

  6. Question 6

    An astronaut's mass is 20 kg. Gravitational field strength on Jupiter is 25 N/kg. What would their weight be there, in N?

    AnswerN

  7. Question 7

    What is the weight of a 50 kg object on Earth? Use g = 9.8 N/kg.

    AnswerN

  8. Question 8

    An astronaut's mass is 80 kg. Gravitational field strength on the Moon is 1.6 N/kg. What would their weight be there, in N?

    AnswerN

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Answer key: Mass and weight (W = mg)

  1. 1. 735 N

  2. 2. 250 N

  3. 3. A. Mass and weight are the same thing.

  4. 4. 64 N

  5. 5. 49 N

  6. 6. 500 N

  7. 7. 490 N

  8. 8. 128 N