Balanced forces cancel out (the net force is zero), so the motion doesn't change. The object stays still or keeps moving at a steady speed. Unbalanced forces change the motion.
A car cruises down the highway at a steady 100 km/h, engine running. Is its engine pushing harder than the air and the road are holding it back?
Last lesson: drawing forces. This lesson: what the diagram tells you about the motion.
Engage: Commit to a prediction before you test anything. A wrong guess is fine: it shows you what to look for.
Explore: Try it on the board and compare what happens with your prediction.
Explain: Use the particle model to explain what you saw.
Practise: A few on your own, to check you have it.
Elaborate: Use the same idea in a new context.
Evaluate: Look back at your first ideas and what changed.
WidthMore contexts, same idea.
Make your prediction first. Help opens once you've locked it in.
EngageCommit to a prediction before you test anything. A wrong guess is fine: it shows you what to look for.
Engage: the cruising car
ExploreTry it on the board and compare what happens with your prediction.
Explore: keep it moving
The box starts moving at 3 m/s. Press Play, then try different pushes (Reset to start again). Watch the speed–time graph and the dots: evenly spaced dots mean a steady speed.
Try it yourselfYour edits are saved in this browser.
Find a push that keeps the box moving at a steady speed for at least 3 seconds.
Use the board. This ticks when you get there.
ExplainUse the particle model to explain what you saw.
Explain: balanced and unbalanced
In your book: explain why "balanced forces" doesn't mean "no forces", using the box on the board as your example.
What happens with each net force? (The box is already moving to the right.)
Push 9 N, friction 6 N
Push 6 N, friction 6 N
Push 2 N, friction 6 N
Push 12 N, friction 6 N
Balanced or unbalanced?
A book resting on a desk
A car cruising at a steady speedSteady speed in a straight line: balanced.
A ball speeding up as it rolls down a slope
A skydiver falling at a steady speedAir resistance equals weight.
A cyclist braking
A plane taking off down the runway
PractiseA few on your own, to check you have it.
Mix it up
Elaborate TypicalUse the same idea in a new context.
Elaborate: the falling skydiver
Try it yourselfYour edits are saved in this browser.
A skydiver and her gear, 100 kg (weight 980 N), fall at a steady speed. Set the air resistance.
Use the board. This ticks when you get there.
Elaborate WiderUse the same idea in a new context.
Elaborate: slowing down and stopping
Try it yourselfYour edits are saved in this browser.
Make the moving box slow down and stop.
Use the board. This ticks when you get there.
EvaluateLook back at your first ideas and what changed.
Evaluate: look back
Write one sentence: "At first I thought… Now I think… because…"
Revision summary
Balanced forces are equal and opposite: the net force is zero. They are still acting; they cancel.
Balanced forces mean no change in motion: still objects stay still; moving objects keep a steady speed in a straight line.
Unbalanced forces change the motion: the object speeds up, slows down or changes direction, the way the net force points.
A car at a steady speed has balanced forces: the engine's push equals air resistance plus friction.
For teachers
Students identify balanced and unbalanced forces and predict the motion, recognising that balanced forces mean no change in motion, not no forces (AC9S7U04, node 2, a hinge node). About 25 minutes.
I can tell balanced from unbalanced forces in a diagram.
I can explain that balanced forces mean no change in motion, not no forces.
I can predict the motion from a force diagram.
Before class
A concept tutorial for introducing or revising the idea; it does not replace trolleys and force meters. Set the width with ?width=minimum, typical (default) or wider.
The hinge misconception is "balanced means no forces" (and its partner, "moving needs a forward force"). The Explore board has motion with balanced forces, which is exactly the case students don't expect.
Board notes: the box moves left and right only. Friction is fixed at its limit while sliding and matches the push when stationary. The board starts the box at 3 m/s so students can find the steady-speed push (6 N).
Running the lesson
Engage: tally the predictions. Most will choose 'forward force is bigger'.
Explore: if students struggle, ask them to compare the push and friction numbers when the graph is flat.
Explain: the first question is the checkpoint; the two sorts apply it.
In your own words: the 'balanced isn't no forces' paragraph is the evidence.
The hard part (on purpose)
Accepting that balanced forces give steady motion, not no motion. When a student says the box should stop, ask what the graph did when push equalled friction.
When students get stuck
A student says the forces are balanced so the box should stop. Ask: What did the graph do when push equalled friction? Fix: It stayed flat: steady speed. Balanced forces keep the motion the same.
A student says there are no forces on the cruising car. Ask: What is the engine doing? What is the air doing? Fix: Both are pushing: forward and backward. They cancel.
A student thinks the skydiver stops when forces balance. Ask: What did the box do when its forces balanced? Fix: It kept moving at the same speed. So does she.
Stretch and support
Stretch: A spacecraft far from any planet switches off its engine. What happens to its motion, and why?
Support: Give the sentence frame: 'The forces are … so the net force is … so the motion …'.
Where it leads
Next: unbalanced forces and changes in motion (node 3), then gravity (node 4). In Year 10 this becomes Newton's first law.
In class: a trolley pulled by a force meter at constant speed (the meter reading equals friction); a tug of war at a standstill.
What each step is for
Engage: the cruising car: Most students say the forward force must be bigger for a car to keep moving; the board shows balanced forces giving steady motion.
Explore: keep it moving: Finding the push that keeps the box at a steady speed shows balanced forces with motion, not without it.
Explain: balanced and unbalanced: Separating 'balanced' from 'no forces', then linking balance to unchanged motion, is the hinge for Newton's first law.
Mix it up: Balanced forces are equal and opposite, which only makes sense once arrow length is read as the size of the force.
Elaborate: the falling skydiver: A vertical case with motion (falling at a steady speed) tests whether students apply 'balanced means no change in motion' beyond the board's horizontal case.
Elaborate: slowing down and stopping: Making the box stop with friction connects unbalanced forces to slowing down and shows what friction does once motion has stopped.
Now you can
Tell balanced from unbalanced forces in a diagram.
Explain that balanced forces mean no change in motion, not no forces.