Teacher sheet · Ages 10–11 · about 45–50 minutes
Balanced and unbalanced forces
Lesson page: echoesof.science/school/forces-and-motion/ages-10-11/ · Pupil worksheet: echoesof.science/school/forces-and-motion/ages-10-11/print/
Learning aims
- Forces have size and direction and are measured in newtons.
- Balanced forces: no change in motion. Unbalanced forces: speeding up, slowing down or turning.
- Forces come in pairs (Newton's third law, informally).
- Plan a fair test, repeat readings and calculate an average.
Lesson plan
| Time | Activity |
|---|---|
| 5 min | Starter: hold a ball still. Which forces act on it? Let it go: what changed? |
| 10 min | Read the text; draw force arrows for the ball and a rolling bike. |
| 20 min | Balloon rocket fair test in groups. |
| 5 min | Play Balanced or unbalanced? |
| 5 min | Quiz and discuss the results. |
Materials for the experiment
- Balloons (latex-free if anyone has a latex allergy)
- A long piece of string (about 5 m)
- A drinking straw
- Sticky tape
- Two chairs
- A measuring tape
- A balloon pump (optional, for hygiene)
Safety: Use a pump or your own balloon only; don't share balloons. Check for latex allergies first. Let go of the balloon away from faces. Don't put balloons or burst pieces in your mouth.
Common misconceptions
| Pupils may think… | Actually… |
|---|---|
| If something is moving, there must be a force pushing it forwards. | A moving object keeps moving at the same speed in a straight line if the forces are balanced. |
| An object at rest has no forces on it. | It can have several forces on it that are balanced, like the ball in your hand. |
| The balloon is pushed by the air behind it in the room. | It is pushed forwards by the air it pushes out backwards. |
Worksheet answers
- An arrow down (gravity) and an equal arrow up (the hand). Yes, they are balanced.
- The balloon pushes the air out backwards; the air pushes the balloon forwards with an equal and opposite force.
- Changed: amount of air (pumps). Measured: distance travelled. Kept the same: balloon type, string, straw, start point.
- Friction between the ball and the grass, and air resistance.
Quiz answers
- In which unit do we measure force? Newtons (N). The SI unit of force is the newton.[3]
- Two equal forces pull a box in opposite directions. What happens? Nothing changes: the forces are balanced. Equal and opposite forces add up to a net force of zero.[1]
- A bike is rolling along and the forces on it are balanced. What does it do? Keeps going at the same speed in a straight line. With balanced forces a moving object keeps the same motion.[2][4]
- Which of these is an unbalanced force making something slow down? Friction on a sliding puck. Friction that slows a body down is an unbalanced force.[5][2]
- A rocket pushes gas out backwards. What does the gas do? Pushes the rocket forwards. The rocket pushes on the gas and the gas pushes on the rocket.[4]
- In the balloon test, why do we do each size three times? To check the results and take an average. Repeats make a result more reliable.
Going further
Tilt the string uphill. Does the balloon still reach the end? Which extra force is now acting against it?
Sources used in this lesson
- [1] Physics, 4.1 Force (OpenStax, Rice University): reliability High
- [2] Physics, 4.2 Newton's First Law of Motion: Inertia (OpenStax, Rice University): reliability High
- [3] Physics, 4.3 Newton's Second Law of Motion (OpenStax, Rice University): reliability High
- [4] Rocket Principles (Newton's laws of motion) (NASA Glenn Research Center): reliability High
- [5] Newton's First and Second Laws of Motion (NASA Glenn Research Center): reliability High
- [7] What is Drag? (NASA Glenn Research Center, Beginner's Guide to Aeronautics): reliability High
Full list with links and a note on each source's reliability: echoesof.science/school/forces-and-motion/sources/
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