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Science lesson plan · 3rd grade (Year 4 in England) · 45 minutes

Balanced and unbalanced forces

Forces and motion

CCSS NGSS.3-PS2-1CCSS NGSS.MS-PS2-2England: KS3 Physics: Motion and forces (balanced forces)England: KS4 Combined Science: Forces

Objective and success criteria

Students will: Balanced and unbalanced forces.

  • 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: Pushes and pulls

Leads to: Gravity; Newton's laws of motion; Which way will it move? Predict from the forces

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 (Pushes and pulls). Students answer on mini whiteboards.
  • Show answers together and note who needs support in the main task.
  1. Is this a push or a pull: opening a drawer? (push / pull) → pullPrompt: Does the thing move away from you or towards you?
  2. Which needs a bigger push to start moving? (an empty shopping trolley (cart) / a feather / a full shopping trolley (cart) / a balloon) → a full shopping trolley (cart)Prompt: Which one is heaviest?
  3. Pushes and pulls: which statement is true? (A gentle push makes a ball go faster than a hard push. / A force can make a still object start to move. / Opening a drawer is a push. / Things can start moving all by themselves with no force.) → A force can make a still object start to move.Prompt: What can a force do to an object?

Hook · 5 min

  • Drop a sheet of paper and a scrunched-up sheet together and ask why one falls faster.
  • Share the objective: Balanced and unbalanced forces. 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: Balanced forces: which statement is NOT true?
  • Step 1: With balanced forces it can move at a steady speed.
  • Step 2: So the answer is: If an object is moving, the forces on it cannot be balanced.
  • Answer: If an object is moving, the forces on it cannot be balanced.
  • 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. Balanced forces: which statement is NOT true? (When forces are balanced, a still object stays still. / A book resting on a table has balanced forces acting on it. / A still object has no forces acting on it. / When forces are balanced, a moving object keeps moving at the same speed in the same direction.) → A still object has no forces acting on it.Prompt: Can something move steadily with balanced forces?

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. Balanced forces: which statement is NOT true? (Forces are measured in kilograms. / When forces are balanced, a moving object keeps moving at the same speed in the same direction. / When forces are balanced, a still object stays still. / Forces are measured in newtons (N).) → Forces are measured in kilograms.Prompt: Can something move steadily with balanced forces?

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: Pushes and pulls.
  • 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: Gravity; Newton's laws of motion; Which way will it move? Predict from the forces.
  • 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-3.

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

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Balanced and unbalanced forces

Name

Date

Score/ 8

  1. Question 1

    A sledge is at rest. It is pulled with 110 N to the left and 40 N to the right. What happens?

    110 N40 N
    • A. it stays still
    • B. it starts to move right
    • C. it starts to move left

  2. Question 2

    A box is pushed with 80 N to the left and 85 N to the right. What is the size of the resultant force, in newtons?

    80 N85 N

    AnswerN

  3. Question 3

    Balanced forces: which statement is NOT true?

    • A. When forces are balanced, a still object stays still.
    • B. Forces are measured in kilograms.
    • C. When forces are balanced, a moving object keeps moving at the same speed in the same direction.
    • D. A book resting on a table has balanced forces acting on it.

  4. Question 4

    A sledge is at rest. It is pulled with 120 N to the left and 50 N to the right. What happens?

    120 N50 N
    • A. it stays still
    • B. it starts to move right
    • C. it starts to move left

  5. Question 5

    A box is pushed with 100 N to the left and 90 N to the right. What is the size of the resultant force, in newtons?

    100 N90 N

    AnswerN

  6. Question 6

    A sledge is at rest. It is pulled with 20 N to the left and 70 N to the right. What happens?

    20 N70 N
    • A. it stays still
    • B. it starts to move right
    • C. it starts to move left

  7. Question 7

    A box is pushed with 95 N to the left and 20 N to the right. What is the size of the resultant force, in newtons?

    95 N20 N

    AnswerN

  8. Question 8

    A sledge is at rest. It is pulled with 50 N to the left and 50 N to the right. What happens?

    50 N50 N
    • A. it stays still
    • B. it starts to move right
    • C. it starts to move left

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Answer key: Balanced and unbalanced forces

  1. 1. C. it starts to move left

  2. 2. 5 N

  3. 3. B. Forces are measured in kilograms.

  4. 4. C. it starts to move left

  5. 5. 10 N

  6. 6. B. it starts to move right

  7. 7. 75 N

  8. 8. A. it stays still