You will learn
- Define unbalanced forces.
- Calculate resultant.
- Link force, mass and acceleration.
- Investigate with trolleys.
Version 0.2 · expanded edition · built from cited public sources Content is based on the official frameworks and sources listed under Sources and standards. No official endorsement is claimed. Written in the style of common primary school assessments. No exam result is promised.
Student lesson
- Unbalanced forces change motion.
- Resultant = bigger minus smaller force in opposite directions.
- Bigger force means bigger acceleration.
- Bigger mass means smaller acceleration.
Words to know
forces that are not equal
change in speed over time
slowing down
the amount of matter
1. Discover: The pushed trolley
Afia pushes a trolley with a small force and it speeds up slowly. She pushes with a bigger force and it speeds up faster. “More force gives more acceleration,” she notes. When she loads the trolley with books it speeds up more slowly.
Think: What did the extra books change?
2. Understand: Unbalanced
Idea 1: Resultant
When unbalanced forces act, the resultant is the difference. 8 N right and 3 N left gives 5 N to the right. The object changes speed or direction.
Idea 2: Acceleration
Acceleration is speeding up; deceleration is slowing down. A bigger resultant force gives bigger acceleration; a bigger mass gives smaller acceleration.
Idea 3: Newton’s second law
This idea is summed up as force = mass × acceleration, a rule you will meet in secondary science.
3. Worked examples
Example 1 (I show you): Calculate
Resultant.
- 8 N right.
- 3 N left.
- Subtract.
Answer: 5 N right.
Example 2 (We do it together): Predict
Mass.
- More mass.
- Less acceleration.
- Same force.
Answer: Slower change.
Example 3 (You try): Explain
Braking.
Did you know?
A rocket accelerates because its engine force is much bigger than gravity on it.
Why this matters
Understanding unbalanced forces helps design safe cars and machines.
4. Explore: Trolley and mass
Pull a trolley with a falling hanging mass over a pulley and time the run for different masses on the trolley and different hanging masses.
- Set up the ramp track.
- Hang a small mass.
- Time the run.
- Vary the mass.
Think about it: what did you notice?
What did you notice? More hanging mass sped it up more.
5. Practise
Say it in a sentence
Use these starters when you talk about the lesson. Say them out loud.
- ___ forces are not equal.
- ___ is change in speed over time.
- ___ is slowing down.
- ___ is the amount of matter.
Guided practice
Question 1. What are unbalanced forces?
Not equal.
Question 2. What is acceleration?
Speeding up.
Question 3. What is deceleration?
Slowing.
Independent practice
Level 1: Start
- What are unbalanced forces?
- What is acceleration?
- What is deceleration?
- How do we find resultant?
- What does more mass do?
Level 2: Grow
- Why speeds up?
- Why mass matters?
- Why friction?
Level 3: Stretch
- Run the trolley.
- Calculate.
- Explain.
Answers are in the answer key below. Try first, then check.
6. Apply: Science in everyday life
Explain what happens to a trolley with 6 N forwards and 2 N back.
Show a sample answer
Calculate the resultant.
Another one. Tell a family member one thing from this lesson. Use the words you learned.
Show a sample answer
A clear sentence that uses at least one word from Words to know.
7. Common mistakes
Fix: Constant acceleration.
Fix: Slower.
8. Lesson summary and revision card
- I can define unbalanced forces.
- I can calculate resultant.
- I can link force, mass and acceleration.
- I can investigate with trolleys.
9. Check and master
Answer all 14 questions. You will see feedback after each one.
Your result appears here after you answer every question.
10. Key takeaways
- Unbalanced forces change motion.
- Resultant = bigger minus smaller force in opposite directions.
- Bigger force means bigger acceleration.
- Bigger mass means smaller acceleration.
11. Next lesson
Next lesson: Gravity. Keep going with the next idea in this course.
Exam practice
These questions are written in the style of common school and international primary assessments. Try them on your own, without looking back. Then check your answers in the marking guide. The marks are shown in brackets.
- Define acceleration and deceleration. [2 marks]
- Calculate resultant of 9 N and 4 N opposite. [2 marks]
- Explain mass and acceleration. [4 marks]
- Safety rule. [2 marks]
Total: 10 marks.
Marking guide and model answers
- 1 each
- Subtract [1]; 5 N in direction of bigger [1]
- Same force [1]; more mass [1]; less acceleration [1]; reason [1]
- Any [1]; reason [1]
Score guide: 9 to 10 = secure. 6 to 8 = nearly there, revise the parts you missed. Under 6 = work through the lesson again with a grown-up.
Printable worksheet
- What are unbalanced forces?
- What is acceleration?
- What is deceleration?
- How do we find resultant?
- What does more mass do?
- Why speeds up?
- Why mass matters?
- Why friction?
- Run the trolley.
- Calculate.
Homework
Predict what happens when you push an empty and a full shopping bag trolley.
Answer key and marking scheme
| Question | Expected response | Marks |
|---|---|---|
| 1. What are unbalanced forces? | Not equal. | 1 |
| 2. What is acceleration? | Speeding up. | 1 |
| 3. What is deceleration? | Slowing. | 1 |
| 4. How do we find resultant? | Subtract. | 1 |
| 5. What does more mass do? | Less acceleration. | 1 |
| 6. Why speeds up? | Resultant forward. | 1 |
| 7. Why mass matters? | Harder to move. | 1 |
| 8. Why friction? | Slows. | 1 |
| 9. Run the trolley. | Table. | 1 |
| 10. Calculate. | Numbers. | 1 |
| Total | 10 |
Independent practice answers. Level 1: 1 Not equal.; 2 Speeding up.; 3 Slowing.; 4 Subtract.; 5 Less acceleration.. Level 2: 1 Resultant forward.; 2 Harder to move.; 3 Slows.. Level 3: 1 Table.; 2 Numbers.; 3 Sentence..
Quiz answers: 1 B, 2 C, 3 A, 4 B, 5 C, 6 A, 7 B, 8 C, 9 A, 10 B, 11 C, 12 A, Challenge 1 B, Challenge 2 C.
Mastery and remediation
| Objective | Bronze (Developing) | Silver (Proficient) | Gold (Advanced) |
|---|---|---|---|
| O1 Define unbalanced forces | Shows this with help from an adult or a picture | Does this independently and explains how they know | Explains it clearly and uses it in a new situation |
| O2 Calculate resultant | Shows this with help from an adult or a picture | Does this independently and explains how they know | Explains it clearly and uses it in a new situation |
| O3 Link force, mass and acceleration | Shows this with help from an adult or a picture | Does this independently and explains how they know | Explains it clearly and uses it in a new situation |
| O4 Investigate with trolleys | Shows this with help from an adult or a picture | Does this independently and explains how they know | Explains it clearly and uses it in a new situation |
Courseversity badge thresholds (overall quiz score): Gold = 90–100%; Silver = 80–89.9%; Bronze = 70–79.9%. A badge is never a full record of progress.
If a learner struggles
- Find the misconception: “A constant force gives constant speed.” Constant acceleration.
- Explain another way: use real objects, pictures or actions first, then words.
- Show a simpler example: Calculate resultant forces.
- Guided practice: work through one example together and let the learner try the next one.
- Reassess: use the quiz again or the worksheet.
- Move on when the learner shows Silver-level evidence independently.
For parents and guardians
In this lesson your child studies unbalanced forces. You do not need to be an expert to help.
- Push a toy car.
- Ask “What if heavier?”
- Clear the floor.
- Ask “how do you know?” after each answer, and listen.
- Praise effort, not only right answers. Mistakes are how we learn.
- Ten to fifteen minutes a day is better than one long session.
Teacher package
1–5. Objectives, prerequisites, materials, preparation, duration
Lesson objectives. Learners will: define unbalanced forces; calculate resultant; link force, mass and acceleration; investigate with trolleys.
Prerequisite knowledge. Grade 5 ideas: pushes, pulls, gravity, friction and air resistance.
Materials. Trolley, pulley, string, masses. Low-resource options: use chalk or charcoal on a board or wall, drawings in the sand, and objects from the classroom instead of printed materials.
Preparation. Set up tracks.
Suggested duration. 45–60 minutes. It can be split into two shorter sessions.
6–9. Teaching procedure, explanation notes, guided and independent activities
Introduction (5 min). Push an empty and loaded toy car and ask, “Which speeds up faster?”
Explanation (10 min). Teach unbalanced forces.
Guided activity (10 min). Learners test.
Independent activity (10 min). Learners calculate.
Assessment and reflection (5 min). Ask each learner to say one thing they learned and how they know it.
Key questions. What is the resultant? How does motion change?
Teacher explanation notes. Start with real objects and safe, hands-on observation. Then talk about what was seen, then draw and write. Always say “how do you know?”. Keep to the safety notes, and use local, safe materials.
10–12. Differentiation, formative assessment, common misconceptions
Support. Use real objects and pictures, short sentences and a partner. Let learners point, show or draw instead of writing. Extension. Ask learners to find a new example at home and explain how they know.
Formative assessment. Listen to learners read and explain, ask “how do you know?”, and check the independent work.
Misconceptions. “A constant force gives constant speed.” Constant acceleration.
13–15. Assessment questions, answer keys, marking scheme
Use the quiz, the 10-mark exam practice and the worksheet on this page. All answers and marking guides are in the answer sections above. Re-check every answer before use.
16–19. Remediation, enrichment, homework, teacher reflection
Remediation. Calculate resultant forces. Enrichment. Look for one new example near your home or school, draw it and explain it to someone. Homework. See above.
Teacher reflection. After teaching, record: which ideas learners understood; which misconceptions occurred; which activities worked; which learners need more practice; and what to improve next time.
Sources and standards
This lesson is based on the sources below, and no official endorsement is claimed. Reference starting points: Next Generation Science Standards, Cambridge Primary Science and the IB Primary Years Programme. Where a standard code is named, it points to the official framework.
Research used to shape this lesson. Courseversity Science curriculum, Grade 6 Unit 6, topic 3; Next Generation Science Standards, forces and interactions; Cambridge Lower Secondary Science, physics; IB Primary Years Programme, how the world works. These sources informed the lesson.